DriveSetup: check if editor view exists in AbstractParametersPanel

* Base class needs to check this as well because the subclass might
  not override it (e.g. InitParametersPanel).

* Fixes crash when trying to initialize partition map. Thanks to
  jessicah for pointing this out.

src/apps/drivesetup/AbstractParametersPanel.cpp

@@ -278,5 +278,6 @@ void
 AbstractParametersPanel::AddControls(BLayoutBuilder::Group<>& builder,
 	BView* editorView)
 {
-	builder.Add(editorView);
+	if (editorView != NULL)
+		builder.Add(editorView);
 }