Displaying an incorrect block preview during dropping a block from sidebar. The preview of a dragged block is the text DROP BLOCK HERE + duplicate of text of following block.

Steps to reproduce

1. click Page
2. click site siteaccess
3. switch to edit mode
4. drag block to the place before other block but don't release it

Expected result

Block preview with text DROP BLOCK HERE.

Actual result

Block preview with text DROP BLOCK HERE + duplication of text of following block.