Protected the null pointer causes cts failed

The current view id is set upon entering and reset to null upon exiting.
The mAssistReceiver may callback after the view exit which caused NPE. To
prevent the symptom, the callback should do nothing when the current view
id is null.

Fixes: 139314597
Fixes: 139349645
Fixes: 139355625
Fixes: 139360077
Test: atest CtsAutoFillServiceTestCases
Test: atest CtsAccessibilityServiceTestCases
Test: atest CtsInputMethodTestCases
Test: atest CtsWidgetTestCases:ToolbarTest

Change-Id: Idbf3c4a24de5547169f905529970e5af795807c2