Fix null-pointer dereference on Android systems when using gl_compatibility renderer.

(cherry picked from commit d186b7438f8ced5e79a7081a29e720d1683cd267)

platform/android/display_server_android.cpp

@@ -676,16 +676,19 @@ void DisplayServerAndroid::cursor_set_custom_image(const Ref<Resource> &p_cursor
 
 void DisplayServerAndroid::window_set_vsync_mode(DisplayServer::VSyncMode p_vsync_mode, WindowID p_window) {
 #if defined(VULKAN_ENABLED)
-	context_vulkan->set_vsync_mode(p_window, p_vsync_mode);
+	if (context_vulkan) {
+		context_vulkan->set_vsync_mode(p_window, p_vsync_mode);
+	}
 #endif
 }
 
 DisplayServer::VSyncMode DisplayServerAndroid::window_get_vsync_mode(WindowID p_window) const {
 #if defined(VULKAN_ENABLED)
-	return context_vulkan->get_vsync_mode(p_window);
-#else
-	return DisplayServer::VSYNC_ENABLED;
+	if (context_vulkan) {
+		return context_vulkan->get_vsync_mode(p_window);
+	}
 #endif
+	return DisplayServer::VSYNC_ENABLED;
 }
 
 void DisplayServerAndroid::reset_swap_buffers_flag() {