Prevent errors if `destroy()` is called twice

Add checks before accessing object properties to see if they hadn't already been set to `null`.

Not modified the package `org.flixel.system.debug.*`
Prevent `destroy()` errors in `org.flixel.system.debug.*`

Now all classes check if an instance is `null` before accessing its members.
Prevent errors if `VCR` or `Vis` are missing the parent

The way `FlxDebugger` is set up, the `parent` will ALWAYS be `null`,
since the instance is removed before as a child before being destroyed.

This is only a  (:::[ ]:::)
Merge pull request #153 from IQAndreas/fix-destroy

Prevent errors if `destroy()` is called twice