Fix relative path error when IFC not under blend dir

When use_relative_path is enabled, Path.relative_to() raises
ValueError if the IFC file is on a different path than the
.blend file. Fall back to the absolute path in that case.

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/bim/handler.py

@@ -445,8 +445,12 @@ def save_post(scene) -> None:
     blend_dir = bpy.path.abspath("//")
     if not blend_dir:
         return
+    from pathlib import Path
     from bonsai.bim.ifc import IfcStore
-    rel_path = os.path.relpath(ifc_path, blend_dir)
+    try:
+        rel_path = str(Path(ifc_path).relative_to(blend_dir))
+    except ValueError:
+        return  # IFC file is not under the blend directory; keep absolute path
     bim_props.ifc_file = rel_path
     IfcStore.set_path(ifc_path)  # keep IfcStore.path absolute for loading
 

src/bonsai/bonsai/bim/ui.py

@@ -94,7 +94,10 @@ class IFCFileSelector:
         filepath = self.get_filepath_abs()
 
         if self.use_relative_path:
-            filepath = filepath.relative_to(bpy.path.abspath("//"))
+            try:
+                filepath = filepath.relative_to(bpy.path.abspath("//"))
+            except ValueError:
+                pass  # IFC file is not under the blend directory; keep absolute path
         return filepath.as_posix().replace("\\", "/")
 
     def draw(self, context: bpy.types.Context) -> None: