CompoundEditor : Fix bug in `editor()` method.
CompoundEditor : Fix bug in `editor()` method.

We need to check if the window exists before asking if its active.

ImageEngine/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,hradec/gaffer,boberfly/gaffer,GafferHQ/gaffer,lucienfostier/gaffer,hradec/gaffer,appleseedhq/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,boberfly/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,boberfly/gaffer,hradec/gaffer,GafferHQ/gaffer,hradec/gaffer,andrewkaufman/gaffer,hradec/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,boberfly/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer