fix USDZ and OBJ export

src/extensions/geo-export/obj-exporter.ts

@@ -145,7 +145,7 @@ export class ObjExporter extends MeshExporter<ObjData> {
                 const color = ObjExporter.getColor(v, geoData, interpolatedColors, interpolatedOverpaint);
                 Color.toArray(color, quantizedColors, i);
             }
-            ObjExporter.quantizeColors(quantizedColors, mesh!.vertexCount);
+            ObjExporter.quantizeColors(quantizedColors, vertexCount);
 
             // face
             for (let i = 0; i < drawCount; i += 3) {

src/extensions/geo-export/usdz-exporter.ts

@@ -149,7 +149,7 @@ def Material "material${materialKey}"
                 const color = UsdzExporter.getColor(v, geoData, interpolatedColors, interpolatedOverpaint);
                 Color.toArray(color, quantizedColors, i);
             }
-            UsdzExporter.quantizeColors(quantizedColors, mesh!.vertexCount);
+            UsdzExporter.quantizeColors(quantizedColors, vertexCount);
 
             // material
             const faceIndicesByMaterial = new Map<number, number[]>();