prevent crash on broken QM parsing

core/models/logbooks.py

@@ -271,12 +271,16 @@ class QM(TroggleModel):
 
     def get_absolute_url(self):
         # This reverse resolution stuff is pure magic. Just change the regex in urls.py and everything changes to suit. Whacky.
+        if self.cave:
+            id = self.cave.slug()
+        else:
+            id = None
         return urljoin(
             settings.URL_ROOT,
             reverse(
                 "qm",
                 kwargs={
-                    "cave_id": self.cave.slug(),
+                    "cave_id": id,
                     "year": self.expoyear,
                     "blockname": self.blockname,
                     "qm_id": self.number,