TD: Fix crash on selecting 3D edge for dimension

Unlike in the bug description of issue 19654 the user doesn't have
to preselect an edge in the 3D view but first start the dimension
command and then select an edge.

This commit adds some security checks to TechDrawHandler::quit(),
TechDrawHandler::getPage() and TDHandlerDimension::onSelectionChanged()
to be on the safe side that no null pointers are dereferenced.

But the ultimative fix for this whole problem is to change
activateHandler() and immediately delete the passed TechDrawHandler if
it fails to find the appropriate QGVPage. This is needed as otherwise
the handler behaves like a ghost object that affects the selection
mechanism and disallows to select anything in the 3D view or the tree
view.

Fixes issue 19654

src/Mod/TechDraw/Gui/CommandCreateDims.cpp

@@ -120,6 +120,7 @@ void positionDimText(DrawViewDimension* dim, int indexOffset = 0);
 
 void activateHandler(TechDrawHandler* newHandler)
 {
+    std::unique_ptr<TechDrawHandler> ptr(newHandler);
     auto* mdi = qobject_cast<MDIViewPage*>(Gui::getMainWindow()->activeWindow());
     if (!mdi) {
         return;
@@ -134,7 +135,7 @@ void activateHandler(TechDrawHandler* newHandler)
     if (!viewPage) {
         return;
     }
-    viewPage->activateHandler(newHandler);
+    viewPage->activateHandler(ptr.release());
 }
 
 //===========================================================================
@@ -517,8 +518,12 @@ public:
             return;
         }
 
+        App::Document* pageDoc = nullptr;
+        if (auto page = getPage()) {
+            pageDoc = page->getDocument();
+        }
         if (msg.Object.getObjectName().empty()
-            || msg.Object.getDocument() != getPage()->getDocument()) {
+            || (msg.Object.getDocument() != pageDoc)) {
             if (msg.Type == Gui::SelectionChanges::AddSelection) {
                 Gui::Selection().rmvSelection(msg.pDocName, msg.pObjectName, msg.pSubName);
             }

src/Mod/TechDraw/Gui/TechDrawHandler.cpp

@@ -109,7 +109,9 @@ void TechDrawHandler::mouseReleaseEvent(QMouseEvent* event)
 
 void TechDrawHandler::quit()
 {
-    viewPage->deactivateHandler();
+    if (viewPage) {
+        viewPage->deactivateHandler();
+    }
 }
 
 QWidget* TechDrawHandler::getCursorWidget()
@@ -127,5 +129,5 @@ void TechDrawHandler::setWidgetCursor(QCursor cursor)
 
 TechDraw::DrawPage* TechDrawHandler::getPage()
 {
-    return viewPage->getDrawPage();
-}
+    return viewPage ? viewPage->getDrawPage() : nullptr;
+}