blockly error on dragging blocks (to edit a sequence)

[tballmsft]

We are able to reproduce an error in blockly (though it takes some doing):

Uncaught NotFoundError: Failed to execute 'insertBefore' on 'Node': The node before which the new node is to be inserted is not a child of this node.
    at ep.setParent (main.js:7:430579)
    at qh.disconnectInternal (main.js:7:321663)
    at qh.disconnectInternal (main.js:7:421838)
    at qh.disconnect (main.js:7:321145)
    at ep.unplugFromStack_ (main.js:7:329043)
    at ep.unplug (main.js:7:328277)
    at r.hideInsertionMarker (main.js:7:495682)
    at r.hidePreview (main.js:7:495336)
    at r.hidePreview (main.js:1:400241)
    at r.previewConnection (main.js:7:493686)

1.Open the program https://makecode.microbit.org/S15045-66568-68684-19044 in https://makecode.microbit.org/beta. It looks like this

3. Now, our goal is to reverse the order of the fill rectangle and draw rectangle blocks just using block drag, so that the draw comes before the fill.

4. First, we drag the fill rectangle block (and all it successors) out, so they are grey.

5. Next we drag the fill circle block down. Notice that if we don't drag it far enough, it clicks back into the fill rectangle block

6. Now we can drag the draw rectangle block above the fill rectangle; do this and swap back and forth. do this while the blocks are disconnected from on start

7. Now drag the connected pair back to on start. Very often the crash will happen when the pair of grey blocks is over on start (before being inserted).