[TD] handle object deletions better

see https://forum.freecadweb.org/viewtopic.php?p=366902#p366902

src/Mod/TechDraw/Gui/ViewProviderTemplate.cpp

@@ -24,6 +24,8 @@
 #include "PreCompiled.h"
 
 #ifndef _PreComp_
+# include <QMessageBox>
+# include <QTextStream>
 # ifdef FC_OS_WIN32
 #  include <windows.h>
 # endif
@@ -47,6 +49,7 @@
 #include <Mod/TechDraw/App/DrawSVGTemplate.h>
 #include <Mod/TechDraw/App/DrawPage.h>
 
+#include "DrawGuiUtil.h"
 #include "QGITemplate.h"
 #include "QGISVGTemplate.h"
 #include "QGVPage.h"
@@ -182,6 +185,31 @@ void ViewProviderTemplate::setMarkers(bool state)
     }
 }
 
+bool ViewProviderTemplate::onDelete(const std::vector<std::string> &)
+{
+    // deleting the template will break the page view, thus warn the user
+
+    // get the page
+    auto page = getTemplate()->getParentPage();
+
+    // generate dialog
+    QString bodyMessage;
+    QTextStream bodyMessageStream(&bodyMessage);
+    bodyMessageStream << qApp->translate("Std_Delete",
+        "The following referencing objects might break.\n\n"
+        "Are you sure you want to continue?\n");
+    bodyMessageStream << '\n' << QString::fromUtf8(page->Label.getValue());
+
+    // show and evaluate dialog
+    int DialogResult = QMessageBox::warning(Gui::getMainWindow(),
+        qApp->translate("Std_Delete", "Object dependencies"), bodyMessage,
+        QMessageBox::Yes, QMessageBox::No);
+    if (DialogResult == QMessageBox::Yes)
+        return true;
+    else
+        return false;
+}
+
 MDIViewPage* ViewProviderTemplate::getMDIViewPage(void) const
 {
     MDIViewPage* myMdi = nullptr;