Custom loop runner

**Bevy version**

0.4.0

**Operating system & version**

Windows 10 and building for WASM

**What you did**

I tried to use a custom runner for Bevy under WASM. I had a simple program that worked until I tried to modify the code use a custom runner as shown in [this sample](https://github.com/bevyengine/bevy/blob/master/examples/app/custom_loop.rs). A repro project [is here](https://github.com/codec-abc/bevy-repro/tree/custom_loop). Be cautious: the master branch compile and run just fine while the ``custom_loop`` branches contains the change to use a custom runner. And while it compiles it crashes at runtime.

**What you expected to happen**

Not to crash when using a custom loop runner.

**What actually happened**

The engine crashed when using a custom

**Additional information**

Nothing really more to say, except the original goal was to have some kind of interop between Seed (a Rust frontend framework) and Bevy. My goal was to add some HTML UI that can drove the engine and vice versa.