Gui: Fix crash in measuring tool after an undo. (#25129)

Co-authored-by: Chris Hennes <chennes@pioneerlibrarysystem.org>

src/Mod/Measure/Gui/TaskMeasure.cpp

@@ -163,6 +163,12 @@ TaskMeasure::TaskMeasure()
     // engage the selectionObserver
     attachSelection();
 
+    if (auto* doc = App::GetApplication().getActiveDocument()) {
+        m_deletedConnection = doc->signalDeletedObject.connect([this](auto&& obj) {
+            onObjectDeleted(obj);
+        });
+    }
+
     if (!App::GetApplication().getActiveTransaction()) {
         App::GetApplication().setActiveTransaction("Add Measurement");
     }
@@ -174,6 +180,7 @@ TaskMeasure::TaskMeasure()
 
 TaskMeasure::~TaskMeasure()
 {
+    m_deletedConnection.disconnect();
     Gui::Selection().setSelectionStyle(SelectionStyle::NormalSelection);
     detachSelection();
 }
@@ -524,6 +531,13 @@ void TaskMeasure::quitMeasurement()
     }
 }
 
+void TaskMeasure::onObjectDeleted(const App::DocumentObject& obj)
+{
+    if (&obj == _mMeasureObject) {
+        _mMeasureObject = nullptr;
+    }
+}
+
 void TaskMeasure::setDeltaPossible(bool possible)
 {
     showDelta->setVisible(possible);

src/Mod/Measure/Gui/TaskMeasure.h

@@ -43,6 +43,8 @@
 #include <Gui/TaskView/TaskView.h>
 #include <Gui/Selection/Selection.h>
 
+#include <boost/signals2/connection.hpp>
+
 namespace MeasureGui
 {
 
@@ -74,6 +76,7 @@ private:
     void setupShortcuts(QWidget* parent);
     void tryUpdate();
     void onSelectionChanged(const Gui::SelectionChanges& msg) override;
+    void onObjectDeleted(const App::DocumentObject& obj);
     void saveMeasurement();
     void quitMeasurement();
 
@@ -87,6 +90,8 @@ private:
     QAction* newMeasurementBehaviourAction {nullptr};
     QToolButton* mSettings {nullptr};
 
+    boost::signals2::connection m_deletedConnection;
+
     void removeObject();
     void onModeChanged(int index);
     void showDeltaChanged(int checkState);