* Provide an error message in case ports of threads are not connected

        For issue #10

src/backends/ocarina-backends-pn-components.adb

@@ -2707,20 +2707,35 @@ package body Ocarina.Backends.PN.Components is
       A_PN_Arc : Node_Id;
       Source_Point : Node_Id;
       Endpoint : Node_Id;
-
+      Ports : List_Id;
       Build_Data_Port : Boolean := Is_Data;
    begin
 
       if Is_In (Port_Instance) then
          --  in port
          Endpoint := Port_Instance;
-         Source_Point := Item
-           (AIN.First_Node (Get_Source_Ports (Port_Instance)));
+         Ports := Get_Source_Ports (Port_Instance);
+         if AINU.Is_Empty (Ports) then
+            Display_Located_Error
+              (AIN.Loc (Port_Instance),
+               "This IN port is not connected to any destination",
+               Fatal => True);
+         else
+            Source_Point := Item (AIN.First_Node (Ports));
+         end if;
+
       else
          --  out port
          Source_Point := Port_Instance;
-         Endpoint := Item
-           (AIN.First_Node (Get_Destination_Ports (Port_Instance)));
+         Ports := Get_Destination_Ports (Port_Instance);
+         if AINU.Is_Empty (Ports) then
+            Display_Located_Error
+              (AIN.Loc (Port_Instance),
+               "This OUT port is not connected to any destination",
+               Fatal => True);
+         else
+            Endpoint := Item (AIN.First_Node (Ports));
+         end if;
 
          --  for out data ports only
          --  to avoid endless fireable transition