Prevent server error in note links when using private proxy (fixes #621)

Author: DavidMStraub

gramps_webapi/api/html.py

@@ -32,7 +32,6 @@
 
 from .util import get_db_handle
 
-
 ALLOWED_TAGS = [
     "a",
     "abbr",
@@ -119,6 +118,8 @@ def build_link(prop: str, handle: str, obj_class: str) -> str:
                 obj = func(ref)
             except HandleError:
                 return ""
+            if not obj:
+                return ""
             gramps_id = obj.gramps_id
         else:
             raise ValueError(f"Unexpected property: {prop}")