GetWorldTimerManager() function can crash if called when no valid World is present

A crash can occur within GetWorldTimerManager() if the function is called when no valid World is present. It is not necessarily intuitive to have to make sure that GetWorld() will not return a nullptr value prior to calling GetWorldTimerManager().

1. Search for the GetWorldTimerManager() function in the Engine's source code.

RESULT:
The function does not validate that the call to GetWorld() returns a valid pointer before calling GetTimerManager(). This can result in a crash if GetWorld() returns a nullptr.

EXPECTED:
The function is protected against GetWorld() returning a nullptr.