Update Extensions.cs (#2310)

- only catching AssemblyResolutionException
- using null propagation instead of null ref

Assets/Mirror/Editor/Weaver/Extensions.cs

@@ -262,9 +262,9 @@ public static IEnumerable<FieldDefinition> FindAllPublicFields(this TypeDefiniti
 
                 try
                 {
-                    typeDefinition = typeDefinition.BaseType.Resolve();
+                    typeDefinition = typeDefinition.BaseType?.Resolve();
                 }
-                catch
+                catch (AssemblyResolutionException)
                 {
                     break;
                 }