BlockFactory is broken

[AnmAtAnm]

Problem statement

BlockFactory sometime fails to initialize with the following error:

blockly_compressed.js:1441 Uncaught TypeError: Cannot read property 'removeChild' of null
    at Blockly.BlockSvg.dispose (blockly_compressed.js:1441)
    at Blockly.WorkspaceSvg.Blockly.Workspace.clear (blockly_compressed.js:947)
    at Blockly.WorkspaceSvg.clear (blockly_compressed.js:1308)
    at Object.FactoryUtils.getDefinedBlock (factory_utils.js:740)
    at BlockExporterTools.createBlockSelectorFromLib (block_exporter_tools.js:216)
    at new BlockExporterController (block_exporter_controller.js:47)
    at new AppController (app_controller.js:48)
    at init (index.html:39)

In my testing, this failed in local file: execution, worked in the staged domain for the August demos, and rarely worked in the normal block factory URL. There is probably a race condition in how the page gets loaded, but that is at all related to the same code running on different domains is very strange.

The same error was seen when attempting to port recent Blockly changes to scratch-blocks.

This appears strongly related to the replacement of goog.dom.removeNode, and the removal of a .parentNode check in that code.

[AnmAtAnm]

First reported here: https://groups.google.com/forum/#!topic/blockly/Ded9E_vrmto
Immediately rolled back demos to June release.