Trying to insert a blank paragraph at the beginning of the document (sometimes?) crashes the editor

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Start by reproducing the crash in the editor using the sequence described in the title: insert a blank paragraph at the beginning of a document, including the cases where it does and does not occur. Trace the resulting error to its source and add a regression test if the project has an established test location; done means the action no longer crashes the editor.

Written by the indexing model from the issue text.