Add null check for input receiver before updating batching

- A change in the system (ie. sysui crash or nav mode change) could
  cause the input monitor to be disposed before the swipe animation
  settles

Bug: 170121063
Test: Kill sysui while swiping up

Change-Id: I1417b109fecdb98fae6197c7038dbe9307470853
diff --git a/quickstep/src/com/android/quickstep/TouchInteractionService.java b/quickstep/src/com/android/quickstep/TouchInteractionService.java
index 7bf8fba..d3c4f55 100644
--- a/quickstep/src/com/android/quickstep/TouchInteractionService.java
+++ b/quickstep/src/com/android/quickstep/TouchInteractionService.java
@@ -732,8 +732,11 @@
     private void reset() {
         mConsumer = mUncheckedConsumer = mResetGestureInputConsumer;
         mGestureState = DEFAULT_STATE;
-        // By default, use batching of the input events
-        mInputEventReceiver.setBatchingEnabled(true);
+        // By default, use batching of the input events, but check receiver before using in the rare
+        // case that the monitor was disposed before the swipe settled
+        if (mInputEventReceiver != null) {
+            mInputEventReceiver.setBatchingEnabled(true);
+        }
     }
 
     private void preloadOverview(boolean fromInit) {