Fix error message on Flicker's launchSplitActivity test

When the Flicker test is unable to launch a split activity it crashes
with a `NullPointerException`.

Change this error into a meaningful error message.

Test: Run `atest WindowManagerSmokeTest`
Change-Id: I1c3c06b3327b94ad6da1a1eabe147182202676a1
1 file changed