Improve commit e0ae646e

Improve commit e0ae646e

> Fix "invokeListener is not a function" uncaught errors

The commit indeed solved the above problem, but apparently replaced it with
another. When `invokeListener` has been `null`ed out, a copy of it can try to
reference things that also have been `null`ed out. So instead of storing copies
of it, we now check if the function exists before calling it. Otherwise, simply
do nothing. This commit also applies this to _all_ places `invokeListener` is
used.