Reset splash icon when task icon is null to avoid caching incorrect drawable.

Fix: 263365662
Test: manual
Change-Id: I79390381f4cab79b9729bd819630eeb9c8b47d1d
diff --git a/quickstep/src/com/android/quickstep/views/TaskThumbnailView.java b/quickstep/src/com/android/quickstep/views/TaskThumbnailView.java
index f6e172a..c71a74e 100644
--- a/quickstep/src/com/android/quickstep/views/TaskThumbnailView.java
+++ b/quickstep/src/com/android/quickstep/views/TaskThumbnailView.java
@@ -377,6 +377,8 @@
 
     private void updateSplashView(Drawable icon) {
         if (icon == null || icon.getConstantState() == null) {
+            mSplashViewDrawable = null;
+            mSplashView = null;
             return;
         }
         mSplashViewDrawable = icon.getConstantState().newDrawable().mutate();