Detached string inputs float loose in the workspace ("potato blocks")

Disconnected string-input shapes appear floating in the workspace ("potato blocks"). They can be typed into but not dragged, and they pop out of the blocks they belong to, particularly custom blocks. The affected blocks often still work at runtime.

Example and diagnosis

A real example: the "load dialogue from compressed string ()" custom block in the NPC sprite of https://scratch.mit.edu/projects/1116676139/ shows a potato block in the top-left of the canvas (reported by @finnagin5).

@nimeratus diagnosed it: there is a text input on the if-else block with the ID uU, which throws an error when Scratch tries to render it. Removing that text input makes the potato block disappear.

• Forum: https://scratch.mit.edu/discuss/topic/880947/

May be the same code path as the custom-block-input-deletion bug (scratchfoundation/scratch-blocks#3652).

Reproduce the custom-block example from project 1116676139 and inspect the rendering path for the text input with ID uU. Compare the behavior with scratchfoundation/scratch-blocks#3652 and the linked forum report. Done means disconnected string inputs no longer float loose while remaining editable and attached to their blocks.