avoid reading deallocated memory

src/states_screens/dialogs/gp_info_dialog.cpp

@@ -246,6 +246,7 @@ void GPInfoDialog::addButtons()
 
 void GPInfoDialog::onEnterPressedInternal()
 {
+    // Save the GP id because dismiss() will destroy this instance
     std::string gp_id = m_gp->getId();
     ModalDialog::dismiss();
     // Disable accidentally unlocking of a challenge

src/states_screens/dialogs/random_gp_dialog.cpp

@@ -138,9 +138,8 @@ GUIEngine::EventPropagation RandomGPInfoDialog::processEvent(
 {
     if (eventSource == "start")
     {
-        // Save the gp since dismiss deletes it otherwise
         ModalDialog::dismiss();
-        race_manager->startGP(m_gp, false, false);
+        race_manager->startGP(grand_prix_manager->m_random_gp, false, false);
         return GUIEngine::EVENT_BLOCK;
     }
     else if (eventSource == "Number of tracks")