desktop: check getDialog()

it might return nullptr

See https://crashreport.libreoffice.org/stats/signature/dp_gui::TheExtensionManager::ToTop()

Change-Id: I4023529aec0ba2c72b4e7db5449d235c208ee30b
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/152268
Tested-by: Jenkins
Tested-by: Caolán McNamara <caolan.mcnamara@collabora.com>
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>

desktop/source/deployment/gui/dp_gui_theextmgr.cxx

@@ -158,7 +158,8 @@ void TheExtensionManager::SetText( const OUString &rTitle )
 {
     const SolarMutexGuard guard;
 
-    getDialog()->set_title( rTitle );
+    if (weld::Window* pDialog = getDialog())
+        pDialog->set_title( rTitle );
 }
 
 
@@ -166,7 +167,8 @@ void TheExtensionManager::ToTop()
 {
     const SolarMutexGuard guard;
 
-    getDialog()->present();
+    if (weld::Window* pDialog = getDialog())
+        pDialog->present();
 }
 
 void TheExtensionManager::Close()