Bug Fix: fix NullPointerException while scrolling message apps

The issue will occur when the view is removed from the hierarchy by
the time the translation response returns. Do a null check to make
sure the view still exists.

Bug: 196933332
Test: atest CtsTranslationTestCases
Tese: manual. not see the crash when scrolling the apps.

Change-Id: Ifee933a802385750d431800ef50f6f2c306ccc0f