Fixed bugs 2981754 and 2981759 (PCBnew crash when trying to add a new net name...

Fixed bugs 2981754 and 2981759 (PCBnew crash when trying to add a new net name to a pad) Linux specific, and perhaps due to a bug in wxGTK 2.8 (does not happen in 2.9)

pcbnew/dialog_pad_properties.cpp

@@ -646,7 +646,7 @@ void DIALOG_PAD_PROPERTIES::PadPropertiesAccept( wxCommandEvent& event )
                     m_CurrentPad->SetNet( net->GetNet() );
                 }
                 else
-                    DisplayError( this, _( "Unknown netname, no change" ) );
+                    DisplayError( NULL, _( "Unknown netname, no change" ) );
             }
         }