Possible fix for lua crash #5269

Check if this instance has been disposed and don't call 'tick'
if it has.

Also remove the finalizer that was broken and wrong anyway.
'runtime' would never become null because it's readonly and
managed resources are freed automatically or may have already
been freed by then.
Merge pull request #5370 from pavlos256/lua-dispose-crash

Possible fix for lua crash #5269