The `tselem->id` pointer can also be used for non-ID data (according to
this comment in DNA_outliner_types.h:
```
/* XXX We actually also store non-ID data in this pointer for identifying
* the TreeStoreElem for a TreeElement when rebuilding the tree. Ugly! */
```
As such, I don't mind adding a `NULL`-check in the
`is_object_data_in_editmode()` function. After all, when there is no
object, its data certainly is not in edit mode.
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