Just a quick fix for USE="gui -debug", haven't spent time on this.
--- a/src/platform/qt/scripting/ScriptingController.cpp
+++ b/src/platform/qt/scripting/ScriptingController.cpp
@@ -87,6 +87,8 @@
 	if (m_controller->hasStarted()) {
 		attach();
+#ifdef ENABLE_DEBUGGERS
 	} else {
 		m_controller->attachDebugger(false);
+#endif
 	}
 	updateVideoScale();