Using press to activate the Tweak tool doesn't work well when used a fallback tool as the drag event is often used by the current tool - making it impossible not to select when dragging (unless the fallback tool is disabled entirely).
Resolve this by using CLICK events when the Tweak tool is used as a fallback.

Even though this avoids the crash, check for null-pointer de-reference since changes to the key-map shouldn't cause operators to crash.

Note that the ability for operators to access a gizmo before it's fully initialized is a more general problem that should be addressed, but out of scope for a bug-fix.