将输入绑定到属性时出现Angular2错误

Question

我有以下HTML

<announcements allowSelecting="true" (selectedItemChanged)="selectedItemChanged($event)"></announcements>

<form [style.visibility]="selectedItem!=undefined |styleVisibility">
    <textarea [(ngModel)]="selectedItem.Message"></textarea>
    <input type="number" step="0" [(ngModel)]="selectedItem.Ordering" />
</form>

基本上这是第一个组件公告，提供一个列表，然后用户可以从该列表中选择该项目，并且selectedItemChanged事件会导致该项目在主要组件中被选中，这就是2个控件上的内容页面绑定到。我知道还有很多与表单有关的内容，在我解决这个问题之后，我将对此进行调查。

我遇到的问题是页面错误地说明了selectedItem是未定义的，当页面首次加载时，它是未定义的。那么解决这个问题的“正确”方法是什么。

标记我考虑过使用ngIf并假设它会起作用，但如果我希望表单可见，在这种情况下我不会，但在其他我可能会这样做。

Gunter 如果我使用？，我会收到不同的错误。句法。在那种情况下，我收到此错误

Error: Uncaught (in promise): Template parse errors:
Parser Error: The '?.' operator cannot be used in the assignment at column 23 in [selectedItem?.Message=$event] in AnnouncementEditComponent@3:14 ("
@
<form [style.visibility]="selectedItem!=undefined |styleVisibility">
    <textarea [ERROR ->][(ngModel)]="selectedItem?.Message"></textarea>
    <input type="number" step="0" [(ngModel)]="selec"): AnnouncementEditComponent@3:14

我最初的方式错误是

EXCEPTION: Error in /app/announcements/announcementEdit.component.html:3:14
ORIGINAL EXCEPTION: TypeError: Unable to get property 'Message' of undefined or null reference
ORIGINAL STACKTRACE:
TypeError: Unable to get property 'Message' of undefined or null reference
   at _View_AnnouncementEditComponent0.prototype.detectChangesInternal (Function code:178:3)
   at AppView.prototype.detectChanges (eval code:12143:13)
   at DebugAppView.prototype.detectChanges (eval code:12247:17)
   at AppView.prototype.detectViewChildrenChanges (eval code:12169:17)
   at AppView.prototype.detectChangesInternal (eval code:12154:13)
   at AppView.prototype.detectChanges (eval code:12143:13)
   at DebugAppView.prototype.detectChanges (eval code:12247:17)
   at AppView.prototype.detectContentChildrenChanges (eval code:12161:17)
   at AppView.prototype.detectChangesInternal (eval code:12153:13)
   at AppView.prototype.detectChanges (eval code:12143:13)