I was not able to reproduce this in UE 4.11.2. Here are the steps I followed: In a Third Person Template press play in editor, Selected Viewport. Press alt + Tab to exit this window and select the UE4ASP_Character BP. Hit stop button in the UE4ASP_CharacterBP. RESULT: Stops in both Main Viewport and Character BP. (I also tried this with Windows docked and undocked as well as with the Character_AnimBlueprint.) -No crashes.
If you have additional information to help reproduce this, please log a bug report on the UE4 Answerhub to the Bug Report section - https://answers.unrealengine.com/spaces/11/bugs-and-crashes.html