Steps to reproduce:

1. Add two paragraphs.
2. Select all and cut.
3. Start a new heading.
4. Paste.

Result: the word "null" appears at the beginning of the pasted content.

Video: https://gfycat.com/ZigzagFlawedIchthyostega /hat tip marctison75 for the report and video posted at https://wordpress.slack.com/archives/C02QB2JS7/p1544019487258300

Video: 25s
^{Tested with WordPress 5.0-RC2-43958 and also with 4.6.1 master @ 874321773 using Firefox 63.0.3 on macOS 10.13.6.}