如何解决尝试在空引用上调用虚拟方法android.content.res.Resources的问题

Question

我正在尝试将quickblox聊天集成到运行良好且已经在Play商店中的现有应用中。

我正在尝试使用示例聊天代码https://github.com/QuickBlox/quickblox-android-sdk/tree/master/sample-chat来实现这一点。

原始应用API的targetSdkVersion为27，由于出现错误，我不得不将其更改为28，并且必须将其迁移到androidX。

因此，在下面的代码块中，loadUsersFromQb（）从服务器向用户加载标签“ SampleTag”。如果存在聊天对话框，则会使用已加载的用户来更新该对话框，否则假定将显示用户的ListView，然后可以选择要添加到聊天组的用户。

代码

 private void loadUsersFromQb() {
        List<String> tags = new ArrayList<>();
        try {
            tags.add("SampleTag");
        }catch (Exception e)
        {
           // Toaster.shortToast(e.getMessage());
        }

        progressBar.setVisibility(View.VISIBLE);
        QBUsers.getUsersByTags(tags, null).performAsync(new QBEntityCallback<ArrayList<QBUser>>() {
            @Override
            public void onSuccess(ArrayList<QBUser> usersByTags, Bundle params) {
                users = usersByTags;
                Toast.makeText(SelectUsersActivity.this, users.toString(), Toast.LENGTH_SHORT).show();



                if (qbChatDialog != null) {
                    // update occupants list form server
                    getDialog();


                } else {
                    updateUsersAdapter();

                }
            }

            @Override
            public void onError(QBResponseException e) {
                Toast.makeText(SelectUsersActivity.this, e.getMessage(), Toast.LENGTH_SHORT).show();
                                     showErrorSnackbar(R.string.select_users_get_users_error, e,
                            new View.OnClickListener() {
                                @Override
                                public void onClick(View v) {
                                    loadUsersFromQb();
                                }
                            });
                    progressBar.setVisibility(View.GONE);

            }
        });
    }

但是

updateUsersAdapter() 

结果

java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources com.gill.XXX.CoreApp.getResources()' on a null object reference

updateUsersAdapter（）块如下所示

private void updateUsersAdapter() {
        usersAdapter = new CheckboxUsersAdapter(this, users);
        if (qbChatDialog != null) {
            usersAdapter.addSelectedUsers(qbChatDialog.getOccupants());
        }
        usersListView.setAdapter(usersAdapter);
        progressBar.setVisibility(View.GONE);
    }

usersListView是从这样的类字段实例化的

private ListView usersListView;

其中ListView是一个Android API 28小部件。

该异常的完整Logcat是

2019-04-04 09:52:07.728 1200-1200/com.gill.XXX:error_activity E/CustomActivityOnCrash: The previous app process crashed. This is the stack trace of the crash:
    java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources com.gill.XXX.CoreApp.getResources()' on a null object reference
        at com.gill.XXX.utils.UiUtils.getCircleColor(UiUtils.java:67)
        at com.gill.XXX.utils.UiUtils.getColorCircleDrawable(UiUtils.java:40)
        at com.gill.XXX.adapter.UsersAdapter.getView(UsersAdapter.java:74)
        at com.gill.XXX.adapter.CheckboxUsersAdapter.getView(CheckboxUsersAdapter.java:41)
        at android.widget.HeaderViewListAdapter.getView(HeaderViewListAdapter.java:220)
        at android.widget.AbsListView.obtainView(AbsListView.java:2365)
        at android.widget.ListView.makeAndAddView(ListView.java:2052)
        at android.widget.ListView.fillDown(ListView.java:786)
        at android.widget.ListView.fillFromTop(ListView.java:847)
        at android.widget.ListView.layoutChildren(ListView.java:1826)
        at android.widget.AbsListView.onLayout(AbsListView.java:2164)
        at android.view.View.layout(View.java:20321)
        at android.view.ViewGroup.layout(ViewGroup.java:6153)
        at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1083)
        at android.view.View.layout(View.java:20321)
        at android.view.ViewGroup.layout(ViewGroup.java:6153)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at android.view.View.layout(View.java:20321)
        at android.view.ViewGroup.layout(ViewGroup.java:6153)
        at androidx.appcompat.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:444)
        at android.view.View.layout(View.java:20321)
        at android.view.ViewGroup.layout(ViewGroup.java:6153)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at android.view.View.layout(View.java:20321)
        at android.view.ViewGroup.layout(ViewGroup.java:6153)
        at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1791)
        at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1635)
        at android.widget.LinearLayout.onLayout(LinearLayout.java:1544)
        at android.view.View.layout(View.java:20321)
        at android.view.ViewGroup.layout(ViewGroup.java:6153)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at com.android.internal.policy.DecorView.onLayout(DecorView.java:763)
        at android.view.View.layout(View.java:20321)
        at android.view.ViewGroup.layout(ViewGroup.java:6153)
        at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2609)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2314)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1450)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7023)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:924)
        at android.view.Choreographer.doCallbacks(Choreographer.java:732)
        at android.view.Choreographer.doFrame(Choreographer.java:664)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:910)
        at android.os.Handler.handleCallback(Handler.java:790)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6523)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:857)

此方法调用导致应用程序崩溃。我在stackoverflow上做了很多类似问题的搜索，但仍然无法解决问题。

在https://issuetracker.google.com/issues/120750246处同样有一个建议，可能是因为我正在API级别小于28的设备上测试该应用程序。ClassNotFoundException也出现在Logcat中。

我已经处理了好几天，任何帮助将不胜感激。