Undo deletion of a custom Block does not add custom Block to toolbox

[mfrtrifork]

Expected Behavior

Custom block is added to toolbox when undo deletion of a custom block.

Actual Behavior

Custom block is NOT added to the toolbox when clicking undo on canvas.

Steps to Reproduce

Add custom block to canvas.
Delete custom block from canvas.
Click undo on canvas.

Operating System and Browser

Mac OS 10.14.4
Google Chrome Version 76.0.3809.100 (Official Build) (64-bit)

[thisandagain]

/cc @picklesrus @paulkaplan @kchadha @cwillisf

[benjiwheeler]

Another demo of the issue, from #2032 :

https://user-images.githubusercontent.com/15828392/72241704-7d779b80-3622-11ea-9791-50c8bfd35a96.gif