broken when rename arguments of function

[duzc2]

Problem statement

when rename and delete all litters of a argument name in function with Input Method, blockly broken.

Expected Behavior

no error

Actual Behavior

Steps to Reproduce

1. Start by create a function with arguments
2. Next, rename a argument name to empty

Stack Traces

Uncaught TypeError: Cannot read property 'name' of null
    at Blockly.BlockSvg.mutationToDom (blocks_compressed.js:92)
    at Blockly.Mutator.workspaceChanged_ (blockly_compressed.js:1321)
    at Blockly.WorkspaceSvg.Blockly.Workspace.fireChangeListener (blockly_compressed.js:953)
    at Blockly.Events.fireNow_ (blockly_compressed.js:901)

Operating System and Browser

Desktop Chrome

Additional Information

[RoboErikG]

Thanks for filing! This is a duplicate of #1958