Fix break on Appel v1.4

When attempting to compile Appel v1.4 (remixed and exported to Appel v1.4 remix.sb3.zip) (I was zipping around several projects to see what works), this error occurs:

TypeError: Cannot read property '0' of undefined
    at /opt/develop/sb-edit/lib/io/leopard/toLeopard.js:893:19
    at Array.map (<anonymous>)
    at _loop_1 (/opt/develop/sb-edit/lib/io/leopard/toLeopard.js:887:93)
    at Project.toLeopard (/opt/develop/sb-edit/lib/io/leopard/toLeopard.js:915:9)
    at /opt/develop/sb-edit/sample/compile.js:8:13

My analysis indicates it's possible that the find call on the variable name entries returns undefined in several spots from the Appel project (75 instances).

This PR is a quick fix that allows the project to compile with sb-edit, but I'm unsure of the consequences of passing the undefined through in the larger context of compiling the app - a compiled Appel v1.4 does not actually run correctly, with graphical problems, and throwing the following error repeatedly:

Uncaught TypeError: Cannot destructure property 'text' of 't' as it is undefined.
    at y._renderBubble (SpeechBubbleSkin.js:87)
    at new y (SpeechBubbleSkin.js:28)
    at w.getSkin (SkinCache.js:53)
    at j.renderSprite (Renderer.js:421)
    at j._renderLayers (Renderer.js:236)
    at j.update (Renderer.js:296)
    at G.render (Project.js:99)
    at G._renderLoop (Project.js:111)

...though, it's entirely possible I'm compiling incorrectly. Here's my compiler code:

const { Project } = require('../lib/index.js');
const { promises: { readFile, writeFile, mkdir } } = require('fs');
const { join, dirname } = require('path');
(async () => {
    const project = await Project.fromSb3(await readFile(join(__dirname, "Appel v1.4 remix.sb3")));
    const tree = project.toLeopard({ printWidth: 100 });
    const tasks = [];
    tasks.push(...Object.keys(tree).map(async (filename) => {
        const content = tree[filename];
        await mkdir(join(__dirname, dirname(filename)), { recursive: true });
        await writeFile(join(__dirname, filename), content);
    }));
    const writeAsset = (owner, type) => async ({ asset, name, ext }) => {
        const filename = join(owner, type, `${name}.${ext}`);
        await mkdir(join(__dirname, dirname(filename)), { recursive: true });
        await writeFile(join(__dirname, filename), new Uint8Array(asset));
    };        
    tasks.push(...project.stage.costumes.map(writeAsset('Stage', 'costumes')));
    tasks.push(...project.stage.sounds.map(writeAsset('Stage', 'sounds')));
    project.sprites.forEach(({ name: spriteName, costumes, sounds }) => {
        tasks.push(...costumes.map(writeAsset(spriteName, 'costumes')));
        tasks.push(...sounds.map(writeAsset(spriteName, 'sounds')));
    });
    await Promise.all(tasks);
    console.log('Done!');
})();

I then attempt to run the output with npx http-serve .; xdg-open http://localhost:8080.

leopard-js / sb-edit

Fix break on Appel v1.4 #58