Add null check for recents scaling during quick switch.

Test: manual - FallbackRecentsTest.goToOverviewFromHome and
StartLauncherViaGestureTests.testStressPressHome were previously failing
due to null mActivity in AbsSwipeUpHandler#getScaleProgressDueToScroll.
Bug: 192470757
Change-Id: I562171c22ad3c89e64b4dc853a266c8b8b78402c
diff --git a/quickstep/src/com/android/quickstep/AbsSwipeUpHandler.java b/quickstep/src/com/android/quickstep/AbsSwipeUpHandler.java
index 830b7fa..8a816ca 100644
--- a/quickstep/src/com/android/quickstep/AbsSwipeUpHandler.java
+++ b/quickstep/src/com/android/quickstep/AbsSwipeUpHandler.java
@@ -1809,7 +1809,7 @@
 
     // Scaling of RecentsView during quick switch based on amount of recents scroll
     private float getScaleProgressDueToScroll() {
-        if (!mActivity.getDeviceProfile().isTablet || mRecentsView == null
+        if (mActivity == null || !mActivity.getDeviceProfile().isTablet || mRecentsView == null
                 || !mRecentsViewScrollLinked) {
             return 0;
         }