VideoThread: Avoid possible null presentation in PresentFrameAndRestoreContext()

src/core/video_presenter.cpp

@@ -1439,6 +1439,8 @@ bool VideoPresenter::PresentFrame(GPUBackend* backend, u64 present_time)
   }
 
   GPUSwapChain* const swap_chain = g_gpu_device->GetMainSwapChain();
+  DebugAssert(swap_chain);
+
   const GPUDevice::PresentResult pres =
     ((backend && !FullscreenUI::IsTransitionActive()) ? RenderDisplay(nullptr, swap_chain->GetSizeVec(), true, true) :
                                                         g_gpu_device->BeginPresent(swap_chain));

src/core/video_thread.cpp

@@ -1014,11 +1014,14 @@ bool VideoThread::Internal::PresentFrameAndRestoreContext()
   if (s_state.gpu_backend)
     s_state.gpu_backend->FlushRender();
 
-  if (!VideoPresenter::PresentFrame(s_state.gpu_backend.get(), 0))
-    return false;
+  if (g_gpu_device->HasMainSwapChain())
+  {
+    if (!VideoPresenter::PresentFrame(s_state.gpu_backend.get(), 0))
+      return false;
 
-  if (s_state.gpu_backend)
-    s_state.gpu_backend->RestoreDeviceContext();
+    if (s_state.gpu_backend)
+      s_state.gpu_backend->RestoreDeviceContext();
+  }
 
   return true;
 }