Fix to no longer default to new graphs being the first DBC signal loaded.

The previous behavior always loaded that first signal and that made graphing custom things more annoying as the signal parameters all had to be deleted first.

graphingwindow.cpp

@@ -521,6 +521,7 @@ void GraphingWindow::showParamsDialog(int idx = -1)
     {
         thisDialog->setParams(graphParams[idx]);
     }
+    else thisDialog->clearParams();
 
     if (thisDialog->exec() == QDialog::Accepted)
     {