QMakePM: fix potential crash when accessing deleted members

Fixes: QTCREATORBUG-28370
Change-Id: I1767678221afc0d1a99194bce2d987a226998da5
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/qmakeprojectmanager/qmakestep.cpp

@@ -552,6 +552,11 @@ QWidget *QMakeStep::createConfigWidget()
             BuildManager::buildLists({bc->cleanSteps()});
     });
 
+    connect(widget, &QObject::destroyed, this, [this] {
+        abisLabel = nullptr;
+        abisListWidget = nullptr;
+    });
+
     VariableChooser::addSupportForChildWidgets(widget, macroExpander());
 
     return widget;