Gui: Fix crash in placement window after closing a document. (#25110)

Fixes #18156

src/Gui/ExpressionBinding.cpp

@@ -57,6 +57,7 @@ void ExpressionBinding::unbind()
 {
     expressionchanged.disconnect();
     objectdeleted.disconnect();
+    documentdeleted.disconnect();
     path = App::ObjectIdentifier();
 }
 
@@ -106,6 +107,8 @@ void ExpressionBinding::bind(const App::ObjectIdentifier &_path)
         expressionchanged = docObj->ExpressionEngine.expressionChanged.connect(std::bind(&ExpressionBinding::expressionChange, this, sp::_1));
         App::Document* doc = docObj->getDocument();
         objectdeleted = doc->signalDeletedObject.connect(std::bind(&ExpressionBinding::objectDeleted, this, sp::_1));
+        documentdeleted = App::GetApplication().signalDeleteDocument.connect(
+            std::bind(&ExpressionBinding::onDocumentDeleted, this, sp::_1));
         //NOLINTEND
     }
 }
@@ -272,6 +275,13 @@ void ExpressionBinding::objectDeleted(const App::DocumentObject& obj)
     }
 }
 
+void ExpressionBinding::onDocumentDeleted(const App::Document& doc)
+{
+    if (path.getOwner() && path.getOwner()->getDocument() == &doc) {
+        unbind();
+    }
+}
+
 // ----------------------------------------------------------------------------
 
 ExpressionWidget::ExpressionWidget() = default;

src/Gui/ExpressionBinding.h

@@ -76,8 +76,10 @@ private:
 protected:
     void expressionChange(const App::ObjectIdentifier& id);
     void objectDeleted(const App::DocumentObject&);
+    void onDocumentDeleted(const App::Document&);
     boost::signals2::scoped_connection expressionchanged;
     boost::signals2::scoped_connection objectdeleted;
+    boost::signals2::scoped_connection documentdeleted;
     bool m_autoApply{false};
 };