我是新手，所以请告诉我在指定详细信息时是否有误。我正在尝试显示一个下拉列表，供用户选择他/她居住的地区。地区名称以及其他详细信息存储在数据库中。我正在尝试创建一个下拉列表，并使用streambuilder从数据库中检索要显示的DropdownMenuItem。 这些是错误日志和代码：-
I/flutter (17707): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter (17707): The following NoSuchMethodError was thrown building StreamBuilder<QuerySnapshot>(dirty, state:
I/flutter (17707): _StreamBuilderBaseState<QuerySnapshot, AsyncSnapshot<QuerySnapshot>>#206f9):
I/flutter (17707): The getter 'value' was called on null.
I/flutter (17707): Receiver: null
I/flutter (17707): Tried calling: value
I/flutter (17707):
I/flutter (17707): When the exception was thrown, this was the stack:
I/flutter (17707): #0 Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:50:5)
I/flutter (17707): #1 new DropdownButton.<anonymous closure> (package:flutter/src/material/dropdown.dart:514:96)
I/flutter (17707): #2 WhereIterator.moveNext (dart:_internal/iterable.dart:439:11)
I/flutter (17707): #3 Iterable.length (dart:core/iterable.dart:423:15)
I/flutter (17707): #4 new DropdownButton (package:flutter/src/material/dropdown.dart:514:112)
I/flutter (17707): #5 _CreateState.test.<anonymous closure> (package:agripreneur/groups/create.dart:71:26)
I/flutter (17707): #6 StreamBuilder.build (package:flutter/src/widgets/async.dart:423:74)
I/flutter (17707): #7 _StreamBuilderBaseState.build (package:flutter/src/widgets/async.dart:125:48)
I/flutter (17707): #8 StatefulElement.build (package:flutter/src/widgets/framework.dart:3809:27)
I/flutter (17707): #9 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3721:15)
I/flutter (17707): #10 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #11 StatefulElement.update (package:flutter/src/widgets/framework.dart:3878:5)
I/flutter (17707): #12 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #13 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #14 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #15 StatelessElement.update (package:flutter/src/widgets/framework.dart:3781:5)
I/flutter (17707): #16 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #17 RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:4585:32)
I/flutter (17707): #18 MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4975:17)
I/flutter (17707): #19 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #20 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #21 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #22 ProxyElement.update (package:flutter/src/widgets/framework.dart:3990:5)
I/flutter (17707): #23 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #24 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #25 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #26 StatefulElement.update (package:flutter/src/widgets/framework.dart:3878:5)
I/flutter (17707): #27 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #28 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #29 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #30 StatefulElement.update (package:flutter/src/widgets/framework.dart:3878:5)
I/flutter (17707): #31 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #32 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #33 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #34 ProxyElement.update (package:flutter/src/widgets/framework.dart:3990:5)
I/flutter (17707): #35 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #36 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #37 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #38 StatefulElement.update (package:flutter/src/widgets/framework.dart:3878:5)
I/flutter (17707): #39 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #40 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4867:14)
I/flutter (17707): #41 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #42 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #43 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #44 StatelessElement.update (package:flutter/src/widgets/framework.dart:3781:5)
I/flutter (17707): #45 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #46 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4867:14)
I/flutter (17707): #47 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #48 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #49 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #50 StatelessElement.update (package:flutter/src/widgets/framework.dart:3781:5)
I/flutter (17707): #51 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #52 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4867:14)
I/flutter (17707): #53 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #54 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #55 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #56 StatefulElement.update (package:flutter/src/widgets/framework.dart:3878:5)
I/flutter (17707): #57 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #58 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #59 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #60 StatefulElement.update (package:flutter/src/widgets/framework.dart:3878:5)
I/flutter (17707): #61 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #62 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4867:14)
I/flutter (17707): #63 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #64 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #65 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #66 StatelessElement.update (package:flutter/src/widgets/framework.dart:3781:5)
I/flutter (17707): #67 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #68 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4867:14)
I/flutter (17707): #69 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #70 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #71 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #72 StatelessElement.update (package:flutter/src/widgets/framework.dart:3781:5)
I/flutter (17707): #73 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #74 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #75 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #76 ProxyElement.update (package:flutter/src/widgets/framework.dart:3990:5)
I/flutter (17707): #77 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #78 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #79 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #80 ProxyElement.update (package:flutter/src/widgets/framework.dart:3990:5)
I/flutter (17707): #81 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #82 RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:4585:32)
I/flutter (17707): #83 MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4975:17)
I/flutter (17707): #84 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #85 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #86 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #87 StatefulElement.update (package:flutter/src/widgets/framework.dart:3878:5)
I/flutter (17707): #88 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #89 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #90 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #91 ProxyElement.update (package:flutter/src/widgets/framework.dart:3990:5)
I/flutter (17707): #92 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #93 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #94 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #95 StatefulElement.update (package:flutter/src/widgets/framework.dart:3878:5)
I/flutter (17707): #96 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #97 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4867:14)
I/flutter (17707): #98 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #99 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #100 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #101 StatelessElement.update (package:flutter/src/widgets/framework.dart:3781:5)
I/flutter (17707): #102 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #103 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:4867:14)
I/flutter (17707): #104 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #105 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): #106 Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter (17707): #107 StatefulElement.update (package:flutter/src/widgets/framework.dart:3878:5)
I/flutter (17707): #108 Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter (17707): #109 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter (17707): Another exception was thrown: NoSuchMethodError: The getter 'value' was called on null.
new StreamBuilder<QuerySnapshot>(
stream: Firestore.instance.collection("location_details").where('state', isEqualTo: 'Maharashtra').snapshots(),
builder: (context,snapshot) {
if (!snapshot.hasData) return new Text("Please wait");
else
return new DropdownButton(
items: snapshot.data.documents.map((document) {
for(int i = 0; i < document['district'].length; i++){
print(i);
print(document['district'][i]);
new DropdownMenuItem(
value: document['district'][i],
child: new Text(document['district'][i]),
).build(context);
i+=1;
}
}
).toList(),
onChanged: (val){
dist = val;
},
hint: new Text("DISTRICT"),
style: TextStyle(color: Colors.black),
);
}
),