Assign net classes to nets before showing the Design Rules dialog

It is needed to display nets that have just been imported to the
board (e.g. in an update PCB from schematics step).

pcbnew/dialogs/dialog_design_rules.cpp

@@ -249,6 +249,9 @@ void DIALOG_DESIGN_RULES::InitDialogRules()
     // Initialize the Rules List
     InitRulesList();
 
+    // Reassure that all nets have net classes assigned
+    m_Pcb->BuildListOfNets();
+
     // copy all NETs into m_AllNets by adding them as NETCUPs.
 
     // @todo go fix m_Pcb->SynchronizeNetsAndNetClasses() so that the netcode==0 is not