🐛 blockIdentifier can be null

[AntoLC]

Describe the bug In a collaborative way, if a user removes the block where another user is located it creates an error.

https://github.com/TypeCellOS/BlockNote/blob/8a5d845407e7bdf63b49fa3b6e4a6d8201e1213a/packages/core/src/editor/BlockNoteEditor.ts#L395

Possible solution

public getBlock(
    blockIdentifier?: BlockIdentifier

Thank you.