The text editor's SelectionController threw 'TypeError: Invalid text
node' when:
- Pressing Backspace to delete the only character of the **first** text
span in a paragraph that contains multiple spans.
- Pressing Delete to delete the only character of the **last** text
span in the same situation.
- Pressing a word-backward shortcut that empties the first span of a
multi-span paragraph.
In all three cases the tree-iterator (previousNode / nextNode) returned
null because no sibling text node existed in that direction, and that
null was subsequently passed to getTextNodeLength() which calls
isTextNode() — which unconditionally throws when given a falsy value.
Fix: use the null-coalescing fallback to the first/last remaining
child's text node of the paragraph before calling collapse() /
getTextNodeLength().
