Error exposed by ba49345705. Code just assumed that the tree-element
pointed to a real ID, but this is often not the case, and the ID pointer
contains completely different data. E.g. before ba49345705, it would
be a pointer to one of the `Main` listbases, so this code would have
undefined behavior. Now the pointer is null for elements in the "Current
File" element, causing a null-pointer dereference rather than undefined
behavior (that just happened to virtually always result in the intended
code path).
4a5cd4e6c7