Bonsai: guard drawing with no document reference in get_sheet_references (#8036)

get_sheet_references calls get_drawing_document, which returns None when the
drawing has no associated IfcRelAssociatesDocument, then compares
reference.Location == drawing_reference.Location, raising
"AttributeError: 'NoneType' object has no attribute 'Location'" during
drawing activation.

A drawing with no document reference cannot be referenced by any sheet, so
return the empty list before the comparison loop. Behavior when a reference
exists is unchanged (the guard is unreachable in that case).

Fixes #8036. Fixes #7266.

Generated with the assistance of an AI coding tool.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/bonsai/bonsai/tool/drawing.py

@@ -2830,6 +2830,10 @@ class Drawing(bonsai.core.tool.Drawing):
     def get_sheet_references(cls, drawing: ifcopenshell.entity_instance) -> list[ifcopenshell.entity_instance]:
         sheet_references: list[ifcopenshell.entity_instance] = []
         drawing_reference = cls.get_drawing_document(drawing)
+        # A drawing with no associated document reference cannot be referenced
+        # by any sheet, so there is nothing to compare against (see #8036, #7266).
+        if drawing_reference is None:
+            return sheet_references
         for sheet in tool.Ifc.get().by_type("IfcDocumentInformation"):
             if not sheet.Scope == "SHEET":
                 continue