fix: Don't throw if drag surface is empty. (#5695)
fix: Don't throw if drag surface is empty. (#5695)

Observed this error in Blockly Games Bird 6 (Chrome OS):
Uncaught TypeError: Failed to execute 'appendChild' on 'Node': parameter 1 is not of type 'Node'.
It has only shown up once, and I can't reproduce, but it looks like the drag surface was being cleared while empty.  Maybe some weird multi-touch operation?

Open question: Leave the code as is so that the error is thown and visible, or supress it?
google/blockly,rachel-fenichel/blockly,mark-friedman/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,google/blockly,google/blockly,mark-friedman/blockly,rachel-fenichel/blockly,mark-friedman/blockly,rachel-fenichel/blockly,google/blockly,google/blockly,mark-friedman/blockly,google/blockly,mark-friedman/blockly,mark-friedman/blockly,google/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly