Fix checkbox delete bug.
Fix checkbox delete bug.

google/blockly,google/blockly,joshlory/blockly,LLK/scratch-blocks,twodee/blockly,groklearning/blockly,rachel-fenichel/blockly,LLK/scratch-blocks,trodi/blockly,mark-friedman/blockly,gengshenghong/scratch-blocks,groklearning/blockly,twodee/blockly,google/blockly,LLK/scratch-blocks,gengshenghong/scratch-blocks,LLK/scratch-blocks,griffpatch/scratch-blocks,google/blockly,picklesrus/blockly,trodi/blockly,mark-friedman/blockly,gengshenghong/scratch-blocks,jstnhuang/blockly,mark-friedman/blockly,LLK/scratch-blocks,rachel-fenichel/blockly,griffpatch/scratch-blocks,nickmain/blockly,nickmain/blockly,picklesrus/blockly,google/blockly,griffpatch/scratch-blocks,rachel-fenichel/blockly,mark-friedman/blockly,picklesrus/blockly,mark-friedman/blockly,jstnhuang/blockly,twodee/blockly,trinketapp/blockly,rachel-fenichel/blockly,groklearning/blockly,jstnhuang/blockly,twodee/blockly,nickmain/blockly,google/blockly,trodi/blockly,rachel-fenichel/blockly,twodee/blockly,twodee/blockly,nickmain/blockly,nickmain/blockly,rachel-fenichel/blockly,joshlory/blockly,google/blockly,nickmain/blockly,griffpatch/scratch-blocks,groklearning/blockly,griffpatch/scratch-blocks,joshlory/blockly,trinketapp/blockly,mark-friedman/blockly,trinketapp/blockly,trinketapp/blockly,gengshenghong/scratch-blocks,jstnhuang/blockly,trodi/blockly,gengshenghong/scratch-blocks,rachel-fenichel/blockly