Fix generic UI crash when plugin has ctrl outputs -- #7937

build_control_ui() special cases `AutomationControl mcontrol = NULL`
to create read-only output display. So Iterating over
Evoral::Control that are not AutomationControl must not crash.

This fixes a bug introduced in a44fecb740.
"Edit with generic controls" caused crashes for plugins with ctrl outs.

gtk2_ardour/generic_pluginui.cc

@@ -359,7 +359,7 @@ GenericPluginUI::build ()
 				= boost::dynamic_pointer_cast<ARDOUR::AutomationControl>(
 					insert->control(param));
 
-			if (c->flags () & Controllable::HiddenControl) {
+			if (c && c->flags () & Controllable::HiddenControl) {
 				continue;
 			}