Deleting block throws error on small screens

[NeilFraser]

• Edit playground's options to simply be:

      {
        trashcan: true,
        toolbox: toolbox
      }

• Open playground in simple mode.
• Open console so that the screen is small.
• Create a stack of two if-else blocks.
• Drag a block from the toolbox to the trash.

In the console is this error:

inject.js:306 Uncaught TypeError: Cannot read property 'getRootBlock' of null
    at bumpObjects (inject.js:306)
    at Blockly.WorkspaceSvg.Blockly.Workspace.fireChangeListener (workspace.js:783)
    at Blockly.Events.fireNow_ (events.js:203)

Affects master and develop. Found on Blockly Games.

[NeilFraser]

Duplicate of #2929