Add null check for mRemoteTargetHandles for side task launch

* Other checks haven't changed, only pulled them out into
separate block

Fixes: 215699962
Test: Unable to repro the crash, I tried ending the
activity and starting via adb shell, but I was never
able to get into the original if-block that triggered the NPE

Change-Id: I83320634f1d059de610176f9031682ca287bd589