This is just not supported, and typically not doable from Blender UI (since embedded IDs won't be listed in ID template list). Add asserts to IDProp code itself, and checks & error reports in relevant BPY and RNA setters.
This is just not supported, and typically not doable from Blender UI (since embedded IDs won't be listed in ID template list). Add asserts to IDProp code itself, and checks & error reports in relevant BPY and RNA setters.