Add leash check in PipTaskOrganizer#fadeExistingPip

Bug: 197319184
Test: N/A, not sable to reproduce the issue, adding protection based on
      the stack trace
Change-Id: I6363dd425eac6332f59fe1fed074c6518227580f

libs/WindowManager/Shell/src/com/android/wm/shell/pip/PipTaskOrganizer.java

@@ -825,6 +825,10 @@ public class PipTaskOrganizer implements ShellTaskOrganizer.TaskListener,
     }
 
     private void fadeExistingPip(boolean show) {
+        if (mLeash == null || !mLeash.isValid()) {
+            Log.w(TAG, "Invalid leash on fadeExistingPip: " + mLeash);
+            return;
+        }
         final float alphaStart = show ? 0 : 1;
         final float alphaEnd = show ? 1 : 0;
         mPipAnimationController