Check if there are passive branche in sub_network during post_processing

pypsa/optimization/optimize.py

@@ -490,15 +490,16 @@ def v_ang_for_(sub):
         buses_i = sub.buses_o
         if len(buses_i) == 1:
             return pd.DataFrame(0, index=sns, columns=buses_i)
-        sub.calculate_B_H(skip_pre=True)
+        if not sub.branches().empty:
+            sub.calculate_B_H(skip_pre=True)
         Z = pd.DataFrame(np.linalg.pinv((sub.B).todense()), buses_i, buses_i)
         Z -= Z[sub.slack_bus]
         return n.buses_t.p.reindex(columns=buses_i) @ Z
 
     # TODO: if multi investment optimization, the network topology is not the necessarily the same,
     # i.e. one has to iterate over the periods in order to get the correct angles.
     # Determine_network_topology is not necessarily called (only if KVL was assigned)
-    if "obj" in n.sub_networks:
+    if ("obj" in n.sub_networks) and not (n.lines.empty):
         n.buses_t.v_ang = pd.concat(
             [v_ang_for_(sub) for sub in n.sub_networks.obj], axis=1
         ).reindex(columns=n.buses.index, fill_value=0.0)