Fix for issue #2707 (#2710)

When selecting the last model of the third model-list in the
model-merging-TUI it crashed because the code forgot about the "None"
element.

Additionally it seems that it accidentally always took the wrong model
as third model if selected?

This simple fix resolves both issues.

ldm/invoke/merge_diffusers.py

@@ -323,7 +323,7 @@ class mergeModelsForm(npyscreen.FormMultiPageAction):
 
         if selected_model3 > 0:
             self.merge_method.values = ['add_difference ( A+(B-C) )']
-            self.merged_model_name.value += f"+{models[selected_model3]}"
+            self.merged_model_name.value += f"+{models[selected_model3 -1]}" # In model3 there is one more element in the list (None). So we have to subtract one.
         else:
             self.merge_method.values = self.interpolations
         self.merge_method.value = 0