Start: Fix possible crash when opening Start page

Procedure to trigger the crash:
* Create a document
* Create a spreadsheet and open it
* Close the 3D view
* Open the Start page
* It may or may not crash but the program is in an undefined state

The reason of the crash is caused by an inappropriate use of static_cast.
This bug is caused by PR 17152.

To fix the crash use dynamic_cast instead. If the cast fails it returns
a null pointer instead of a dangling pointer.

src/Mod/Start/Gui/StartView.cpp

@@ -417,8 +417,7 @@ void StartView::changeEvent(QEvent* event)
     _openFirstStart->setEnabled(true);
     Gui::Document* doc = Gui::Application::Instance->activeDocument();
     if (doc) {
-        Gui::View3DInventor* view = static_cast<Gui::View3DInventor*>(doc->getActiveView());
-        if (view) {
+        if (auto view = dynamic_cast<Gui::View3DInventor*>(doc->getActiveView())) {
             Gui::View3DInventorViewer* viewer = view->getViewer();
             if (viewer->isEditing()) {
                 _openFirstStart->setEnabled(false);