Uncaught TypeError: Cannot read properties of undefined (reading 'neighbors')

[zepumph]

Hit this while testing in #369:

http://localhost:8080/buoyancy/buoyancy_en.html?brand=phet-io&phetioStandalone&ea&debugger&preferencesStorage&fuzz&fuzzBoard&disableModals

I don't think we need to do much here, just want to track it in case it starts showing up more.

p2-0.7.1.js:12276 Uncaught TypeError: Cannot read properties of undefined (reading 'neighbors')
    at IslandManager.split (p2-0.7.1.js:12276:12)
    at World.internalStep (p2-0.7.1.js:13093:27)
    at World.step (p2-0.7.1.js:12895:18)
    at PhysicsEngine.step (PhysicsEngine.ts:119:16)
    at BuoyancyShapesModel.step (DensityBuoyancyModel.ts:398:17)
    at Sim.ts:444:22
    at PhetioAction.execute (PhetioAction.ts:162:17)
    at Sim.stepSimulation (Sim.ts:1068:31)
    at Sim.stepOneFrame (Sim.ts:1058:12)
    at Sim.runAnimationLoop (Sim.ts:1033:12)

[samreid]

Relevant code in the area:

    // Add connectivity data. Each equation connects 2 bodies.
    for(var k=0; k!==equations.length; k++){
        var eq=equations[k],
            i=bodies.indexOf(eq.bodyA),
            j=bodies.indexOf(eq.bodyB),
            ni=nodes[i],
            nj=nodes[j];
        ni.neighbors.push(nj);
        nj.neighbors.push(ni);
        ni.equations.push(eq);
        nj.equations.push(eq);
    }

[zepumph]

I believe we have a reproducible case over in #399 (comment):

After Reset All, if you try to select "same volume" or a different shape on the shapes screen with the mouse this console error repeatedly pops up: