Fix NPE that happens occasionally on start-up

Was not checking if topActivity was null.

Bug: 189935087
Test: boot phone thousands of times.
Change-Id: I3c779be5f09d82d52a3df9478c801a1963e0a5c2

quickstep/src/com/android/launcher3/QuickstepTransitionManager.java

@@ -1124,6 +1124,7 @@ public class QuickstepTransitionManager implements OnDeviceProfileChangeListener
             if (target.mode == mode && target.taskInfo != null
                     // Compare component name instead of task-id because transitions will promote
                     // the target up to the root task while getTaskId returns the leaf.
+                    && target.taskInfo.topActivity != null
                     && target.taskInfo.topActivity.equals(mLauncher.getComponentName())) {
                 return true;
             }