'this' always returns nullptr in member functions called from another class instance.

Whenever a class/script instance tries to invoke a member function belonging to another class instance, the implicit 'this' pointer always returns nullptr like if the method was run as a static function.

As a result of this, whenever a class attribute is referenced in the invoked member function, a read access violation exception is thrown, crashing the game.

Example:
```
void FooNode::_process(float delta)
{
// thing is a reference to a ThingNode2D node instance.
thing->do_stuff();
}

void ThingNode2D::do_stuff()
{
attribute = 5;
}
```
This crashes when the member variable 'attribute' is read.