Bug: AttributeError: 'TagifiedTag' object has no attribute 'insert' #2259

Ex error output:

2026/05/21 4:42:49 PM:   File "/app/python/env/lib/python3.13/site-packages/shiny/_app.py", line 486, in _render_page
2026/05/21 4:42:49 PM:     ui_res.insert(
2026/05/21 4:42:49 PM:     ^^^^^^^^^^^^^
2026/05/21 4:42:49 PM: AttributeError: 'TagifiedTag' object has no attribute 'insert'

From: https://github.com/posit-dev/py-htmltools/blob/main/CHANGELOG.md#070---2026-05-21

Breaking changes

• Tagifiable.tagify()'s return annotation is now Tagified, a new type alias for the union of fully-tagified shapes (TagifiedNode | float | None | Sequence[Tagified], mirroring TagChild). Custom .tagify() implementations annotated with bare TagList / Tag return types should switch to -> Tagified, or drop the annotation. Runtime behavior of correct implementations is unchanged. (Make Tag/TagList generic in their child type so .tagify() can statically guarantee a fully-tagified tree py-htmltools#105, Investigate: make TagifiedTagList a real subclass instead of a type alias py-htmltools#116)

• The result of .tagify() is now immutable. Calling .append, .extend, .insert, .add_class, __setitem__, or the context-manager with form on a tagified value raises AttributeError and is a static type error. Mutate on the buildable Tag / TagList side, then call .tagify() once to produce the render-ready result. (Investigate: make TagifiedTagList a real subclass instead of a type alias py-htmltools#116)

• TagList.tagify() now raises TypeError at the boundary when a child's .tagify() returns un-tagified content (e.g. a bare TagList containing a still-Tagifiable object). The error names the offending class and slot index so buggy .tagify() implementations surface at the source. The render-time RuntimeError for the same family of violations has been clarified. (Unable to render tagifiable class that returns a TagList py-htmltools#7, Make Tag/TagList generic in their child type so .tagify() can statically guarantee a fully-tagified tree py-htmltools#105, feat: TagList.tagify() raises TypeError on un-tagified content py-htmltools#112, Investigate: make TagifiedTagList a real subclass instead of a type alias py-htmltools#116)

Fix

• Please require the latest shiny release: shiny >= 1.6.2
• Or restrict your htmltools version: htmltools < 0.7.0

Fixed in #2244