Measure: Fix TaskMeasure

Made several fixes to TaskMeasure:
* Move to correct namespace
* Handle possible exception in update() method
* Add null pointer checks for buttonBox
* Cannot use 'Measure::MeasurePython' as template argument in addObject<>
  because the macro PROPERTY_HEADER_WITH_OVERRIDE determines the invalid
  string 'App::FeaturePythonT<FeatureT>' so that an exception is raised

Note: The changes might fix issue 20304

src/Mod/Measure/Gui/Command.cpp

@@ -62,7 +62,7 @@ void StdCmdMeasure::activated(int iMsg)
 {
     Q_UNUSED(iMsg);
 
-    Gui::TaskMeasure* task = new Gui::TaskMeasure();
+    MeasureGui::TaskMeasure* task = new MeasureGui::TaskMeasure();
     task->setDocumentName(this->getDocument()->getName());
     Gui::Control().showDialog(task);
 }