BC: Fix combobox delegate.

src/Model/Network/Graph.py

@@ -51,12 +51,17 @@ class Graph(object):
         )
 
     def node(self, node_name:str):
-        return list(
+        node =  list(
             filter(
                 lambda n: n.name == node_name,
                 self._nodes
             )
-        )[0]
+        )
+
+        if len(node) == 0:
+            return None
+
+        return node[0]
 
     def _add_node(self, node):
         self._nodes.append(node)