add null check to monaco toolbox drag logic (#3723)
add null check to monaco toolbox drag logic (#3723)


Microsoft/pxt,switch-education/pxt,playi/pxt,Microsoft/pxt,switch-education/pxt,switch-education/pxt,Microsoft/pxt,switchinnovations/pxt,playi/pxt,playi/pxt,switchinnovations/pxt,playi/pxt,switch-education/pxt,switchinnovations/pxt,switch-education/pxt,Microsoft/pxt,switchinnovations/pxt,Microsoft/pxt