Fix NullPointerException

There could be no running activity in the Task if all activities
are finishing.

The exception seems only happening on monkey tests where the touch
events send randomly while the activities are finishing.

Bug: 362379175
Test: wm presubmit
Flag: EXEMPT bugfix
Change-Id: I5277f9e16e036fcab146fb693fd033d22dc9248a
1 file changed