RayTracer.dispose() throws when a solver is deleted before it has run

[rdmiller]

Deleting a Ray Tracer that has never been run throws, and the delete does not complete.

Reproduce

1. Open the CRAM editor
2. Solvers → Raytracer (adds a solver)
3. Click the trash icon next to it without pressing Run

Error

TypeError: Cannot convert undefined or null to object
    at Object.keys (<anonymous>)
    at RayTracer.dispose (compute/raytracer/index.ts:435)
    at emit
    at onDelete

Cause

RayTracer.dispose() does:

// src/compute/raytracer/index.ts:435
Object.keys(window.vars).forEach(key => {
  if (window.vars[key]['uuid'] === this.uuid) {
    delete window.vars[key];
  }
})

window.vars is only created lazily by registerGlobalVar (src/common/global-vars.ts:7), which a Ray Tracer reaches during a run. Delete a solver that never ran and window.vars is still undefined, so Object.keys(undefined) throws.

Because the throw happens partway through dispose(), the two lines after it never execute either:

renderer.scene.remove(this.rays);
renderer.scene.remove(this.hits);

so the solver's ray and hit objects are left attached to the scene.

Suggested fix

Guard the lookup (window.vars ?? {}), or initialise window.vars once at module load in common/global-vars.ts so every consumer can assume it exists.

Notes

Pre-existing and unrelated to any current PR — found incidentally while testing #87. Reachable with no geometry involved: adding a solver and immediately deleting it is enough.