Error encountered when inserting multiple columns into multiple columns

[freesuton]

Describe the bug
When inserting a multi-column in a multi-column, it shows

To Reproduce
[<clear steps to reproduce are super helpful! Best is to provide an online sandbox, click to create one>
](https://www.blocknotejs.org/examples/basic/multi-column)

Try in the multi-column page.

[matthewlipski]

I think we probably want to change this to only allow columnLists at the top level, so not nested within other blocks or columns (and have the slash menu reflect that). Right now it's also still possible to create them as children of blocks.

[YousefED]

I was not able to reproduce this. @matthewlipski @freesuton let me know if you can

[matthewlipski]

Opening the slash menu at the end of "So is this heading!" in the example and selecting the "Two columns" option throws an error for me. It is a different error, but still related to inserting multiple columns into multiple columns.

[matthewlipski]

Error in question

[YousefED]

It's because according to the prosemirror schema, nested columns are not allowed. However, the slash menu command does not account for this.

My proposed fix would be to insert the new columnlist below the existing ones, whenever the slash menu command is triggered from within a column