Function nodes do not reset or show warning when changing its function owner to a different type

[jdaignault]

It is possible for a binding to try and call a function that doesn’t exist for its function owner type, by swapping its type to another after having already selected a function.

Steps to reproduce:

1. Create a widget with an Image and a TextBlock, and ensure that both are exposed as variables
2. Create a binding with a Read a Property node for the Image
3. Connect its output to a Call a Function destination node, and set its function to Set Color and Opacity
4. Create another Read a Property node for the TextBlock
5. Connect its output to replace the Function Owner parameter of the Call a Function node
6. Observe that no errors are displayed, and that the blueprint saves/compiles without warning
7. Add the widget to the HUD and run PIE
8. Observe that the engine crashes, with a callstack implying that it tries to invoke SImage::SetColorAndOpacity on the TextBlock