Speck component rendering fix

[shammamah-zz]

Closes #310

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• This is a new component
• I am adding a feature to an existing component, or improving an existing feature
• I am closing an issue

Description of changes

This issue was due to the fact that loadStructure was not being called from componentDidMount, and instead only from componentDidUpdate. I also ran into an issue where loadStructure failed because this.state.renderer was still null, even after having defined it -- this was because setState does not immediately mutate the state of the component (https://reactjs.org/docs/react-component.html#setstate). From the documentation:

setState() does not always immediately update the component. It may batch or defer the update until later. This makes reading this.state right after calling setState() a potential pitfall. Instead, use componentDidUpdate or a setState callback (setState(updater, callback)), either of which are guaranteed to fire after the update has been applied. If you need to set the state based on the previous state, read about the updater argument below.

I therefore added a callback to call loadStructure() only after the renderer had been updated to a non-null value.

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