Fixed crash with a custom `MainLoop` or with `--check-only` #310

Job	Run time
⛏ Data preparation	8s
🕸 Build a Web library (web, template_release, wasm32, web.demo_build, lto=yes force_enabled_dd3d...	3m 45s
📚 Generate Docs	21s
🕸 Build a Web library (linux, editor, x86_64, linux.demo_build, lto=yes)	1m 12s
🌐📦 Export Demo Project	1m 4s
🏁 Finalization	10s
🌐 Deploy	43s
🔄 Restore gh-pages	0s
	7m 23s

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