Panic when rapidly clicking on platforms in plasticity 2D web demo

In the 2D plasticity web demo, rapidly clicking on any of the platforms causes a panic.
```
Panic error message: called `Option::unwrap()` on a `None` value
__cargo_web_snippet_97495987af1720d8a9a923fa4683a7b683e3acd6
_ZN6stdweb7webcore14initialization10initialize28_$u7b$$u7b$closure$u7d$$u7d$17he157f34f8c851715E
_ZN3std9panicking20rust_panic_with_hook17h868a29d5aa6e3f6fE
_ZN3std9panicking18continue_panic_fmt17h5bc0f5dc42367f4bE
rust_begin_unwind
_ZN4core9panicking9panic_fmt17h52bd9c4c06b66d8dE
_ZN4core9panicking5panic17h1fb303f1c113605dE
_ZN85_$LT$nphysics_testbed2d..testbed..Testbed$u20$as$u20$kiss3d..window..state..State$GT$4step17h8f216363b0a9a2dbE
_ZN229_$LT$stdweb..webcore..newtype..Newtype$LT$$LP$stdweb..webcore..serialization..FunctionTag$C$$u20$$LP$A12$C$$RP$$RP$$C$stdweb..webcore..once..Once$LT$F$GT$$GT$$u20$as$u20$stdweb..webcore..serialization..FuncallAdapter$LT$F$GT$$GT$15funcall_adapter17hfa87527c98056e7dE
Module.STDWEB_PRIVATE.dyncall
output
```
Everything in the demo is at default settings. Replicated in Firefox 60.8.0esr and Chrome 78.0.3904.87 on Windows 10.