Web flamegraph sometimes crashes with "startsWith of undefined" (rack-mini-profiler)

When used to view traces recorded by `rack-mini-profiler`, the web-based flamegraph sometimes crashes with the following error

> speedscope.44364064.js:174 Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'startsWith')

The minified code is
```
for(let t of this.frames)t.name.startsWith("__Z")&& ...
```
which appears to be the call on [this line](https://github.com/jlfwong/speedscope/blob/b71cef5db4162e9cd871ad7a1bf5716fffabe5ff/src/lib/profile.ts#L409)