Weird renderer bug: Cannot read property 'parentNode' of undefined

[Reinmar]

This is an umbrella ticket for the below listed tickets:

• Occasionally error when Move range between different roots. #5601
• There should be no errors when indenting a code block  #5910
• Editor crashes after pasting the content multiple times #1579
• When there is a lot of content in root,paste other content which's first element is a picture. Error. #5843
• Renderer#_updateChildren fails when DOM element is "moved back" #6367

It manifests itself with:

remove.js:16 Uncaught TypeError: Cannot read property 'parentNode' of undefined
    at remove (remove.js:16)
    at Renderer._updateChildren (renderer.js:557)
    at Renderer.render (renderer.js:203)
    at View._render (view.js:682)
    at View.<anonymous> (view.js:187)
    at View.fire (emittermixin.js:209)
    at View.change (view.js:479)
    at View._disableRendering (view.js:669)
    at Model.EditingController.listenTo.priority (editingcontroller.js:84)
    at Model.fire (emittermixin.js:209)

And was investigated by @oleq in #5910.

I'll close other tickets as DUP of this one, but after closing this one we should review all of them too.

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[Reinmar]

More investigations: #1579 (comment).

[Reinmar]

Another issue: #6367.