get_tree().reload_current_scene() causes at("") to crash game

[jgrenda]

Using get_tree().reload_current_scene() to reload the game causes at("") to no longer work and give the following error in the stack trace: "Attempt to call function 'at' in base 'previously freed' on a null instance".

Possible fix:
Include validity check when returning things from cache in U.next_store.
if key in _at_cache: return _at_cache[key] becomes
if key in _at_cache and is_instance_valid(_at_cache[key]): return _at_cache[key]