Backends: DX10, DX11: avoid ImGui_ImplXXXX_SwapBuffers() handlers for secondary viewports crashing if SwapChain could not be created.

It's not really clear when that would fail but someone privately reported, and since we allow failure in ImGui_ImplDX11_CreateWindow() it makes sense to allow it here.

backends/imgui_impl_dx10.cpp

@@ -770,7 +770,8 @@ static void ImGui_ImplDX10_RenderViewport(ImGuiViewport* viewport, void*)
 static void ImGui_ImplDX10_SwapBuffers(ImGuiViewport* viewport, void*)
 {
     ImGui_ImplDX10_ViewportData* vd = (ImGui_ImplDX10_ViewportData*)viewport->RendererUserData;
-    vd->SwapChain->Present(0, 0); // Present without vsync
+    if (vd->SwapChain)
+        vd->SwapChain->Present(0, 0); // Present without vsync
 }
 
 void ImGui_ImplDX10_InitMultiViewportSupport()

backends/imgui_impl_dx11.cpp

@@ -800,7 +800,8 @@ static void ImGui_ImplDX11_RenderWindow(ImGuiViewport* viewport, void*)
 static void ImGui_ImplDX11_SwapBuffers(ImGuiViewport* viewport, void*)
 {
     ImGui_ImplDX11_ViewportData* vd = (ImGui_ImplDX11_ViewportData*)viewport->RendererUserData;
-    vd->SwapChain->Present(0, 0); // Present without vsync
+    if (vd->SwapChain)
+        vd->SwapChain->Present(0, 0); // Present without vsync
 }
 
 static void ImGui_ImplDX11_InitMultiViewportSupport()

docs/CHANGELOG.txt

@@ -142,6 +142,8 @@ Docking+Viewports Branch:
   backends have been shutdown since 1.90.4. Changed into asserts. (#7175, #8945)
 - Backends: DX10, DX11, DX12: Disabled DXGI's Alt+Enter default behavior on secondary
   viewports managed by the backend. (#4350) [@PathogenDavid]
+- Backends: DX10, DX11: avoid ImGui_ImplXXXX_SwapBuffers() handlers for secondary
+  viewports crashing if SwapChain could not be created.
 - Backends: Vulkan: Added a way to configure secondary viewport pipelinen creation
   by setting init_info.PipelineInfoForViewports fields. (#8946, #8110, #8111, #8686)
 - Backends: Vulkan: Added a way to configure secondary viewport swapchain VkImageUsageFlags