added some checks for null plugs in Box's plug promotion methods
added some checks for null plugs in Box's plug promotion methods

cedriclaunay/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,appleseedhq/gaffer,cedriclaunay/gaffer,chippey/gaffer,goddardl/gaffer,lucienfostier/gaffer,davidsminor/gaffer,hradec/gaffer,ivanimanishi/gaffer,appleseedhq/gaffer,goddardl/gaffer,hradec/gaffer,cedriclaunay/gaffer,boberfly/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,goddardl/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,goddardl/gaffer,ImageEngine/gaffer,ivanimanishi/gaffer,hradec/gaffer,appleseedhq/gaffer,hradec/gaffer,davidsminor/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,ivanimanishi/gaffer,chippey/gaffer,boberfly/gaffer,johnhaddon/gaffer,davidsminor/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,ImageEngine/gaffer,boberfly/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,chippey/gaffer,boberfly/gaffer,appleseedhq/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,cedriclaunay/gaffer,GafferHQ/gaffer,chippey/gaffer,ivanimanishi/gaffer,davidsminor/gaffer,hradec/gaffer