fix crash on minimize

src/main.cpp

@@ -898,6 +898,13 @@ int main(int argc, char **argv) {
             }
         }
 
+        if (!swapchain_texture) {
+            SDL_CancelGPUCommandBuffer(command_buffer);
+            ImGui::Render();
+            ImDrawData *draw_data = ImGui::GetDrawData();
+            continue;
+        }
+
         per_frame.aspect_ratio = (float) window_width / (float) window_height;
         SDL_PushGPUVertexUniformData(command_buffer, 0, &per_frame, sizeof(per_frame));