Add null check in #animateViewIn to guard against the post runnable executing after the view has been destroyed.

Resolves #917

PiperOrigin-RevId: 290300667
(cherry picked from commit bd7de3d)

Author: wcshi

lib/java/com/google/android/material/snackbar/BaseTransientBottomBar.java

@@ -263,6 +263,9 @@ public boolean handleMessage(@NonNull Message message) {
       new Runnable() {
         @Override
         public void run() {
+          if (view == null || context == null) {
+            return;
+          }
           // Calculate current bottom inset, factoring in translationY to account for where the
           // view will likely be animating to.
           int currentInsetBottom =
@@ -845,6 +848,9 @@ void animateViewIn() {
         new Runnable() {
           @Override
           public void run() {
+            if (view == null) {
+              return;
+            }
             // Make view VISIBLE now that we are about to start the enter animation
             view.setVisibility(View.VISIBLE);
             if (view.getAnimationMode() == ANIMATION_MODE_FADE) {