Selected Trigger is not cleared in the TriggerWindowManager when new triggers are loaded

[chreden]

_selectedTrigger in TriggerWindowManager is not cleared when the triggers are reloaded, so the pointer outlives the actual object being pointed to. This is sort of a problem with it being a pointer - Item doesn't have this problem.

Repro Steps

1. Open a level
2. Select a trigger
3. Open a new level
4. Open a triggers window
5. Sometimes it will crash

Selected trigger needs to be cleared whenever the triggers list is set.

[chreden]

Long term fix for this is #752.