Handle missing effect bands on multi voice effect scenarios.

src/rendering/EffectBand.ts

@@ -110,10 +110,13 @@ export class EffectBand extends Glyph {
                             // load the effect from the previous renderer if possible.
                             let previousRenderer: EffectBarRenderer = this.renderer
                                 .previousRenderer as EffectBarRenderer;
-                            let previousBand: EffectBand = previousRenderer.getBand(this.voice, this.info.effectId)!;
-                            let voiceGlyphs: Map<number, EffectGlyph> = previousBand._effectGlyphs[b.voice.index];
-                            if (voiceGlyphs.has(prevBeat.index)) {
-                                prevEffect = voiceGlyphs.get(prevBeat.index)!;
+                            let previousBand = previousRenderer.getBand(prevBeat.voice, this.info.effectId);
+                            // it can happen that we have an empty voice and then we don't have an effect band 
+                            if(previousBand) {
+                                let voiceGlyphs: Map<number, EffectGlyph> = previousBand._effectGlyphs[prevBeat.voice.index];
+                                if (voiceGlyphs.has(prevBeat.index)) {
+                                    prevEffect = voiceGlyphs.get(prevBeat.index)!;
+                                }
                             }
                         }
                         // if the effect cannot be expanded, create a new glyph

test/visualTests/issues/BrokenRenders.test.ts

@@ -0,0 +1,7 @@
+import { VisualTestHelper } from "../VisualTestHelper";
+
+describe('BrokenRendersTests', () => {
+    it('let-ring-empty-voice', async () =>{
+        await VisualTestHelper.runVisualTest('issues/let-ring-empty-voice.gp');
+    })
+})