Pixi-Spine and Spine v3.5.51

Hello,

I'm attempted to load a spine animation (v3.5.51) into the current version of pixie-spine (as it states that it's only compatible with Spine 3.5.x

I'm not using the Pixi loader, instead I'm just passing in the loaded Spine JSON in the constructor.  It's valid JSON with the bones/animation properties in there (so not a string etc)

However, what I'm getting is a crash 'Cannot read property 'children' of undefined at new Skeleton':

 for (var i = 0; i < data.bones.length; i++)
            {
                var boneData = data.bones[i];
                var bone = void 0;
                if (boneData.parent == null)
                    bone = new core.Bone(boneData, this, null);
                else
                {
                    var parent_1 = this.bones[boneData.parent.index];
                    bone = new core.Bone(boneData, this, parent_1);
                    parent_1.children.push(bone); <-- ERROR/CRASH HERE
                }
                this.bones.push(bone);
            }

The exported JSON is v3.5.51:

"skeleton": { "hash": "m3kYx9f3hy1R9hSrlYyJs6o/0fk", "spine": "3.5.51", "width": 202.11, "height": 317.08, "images": "" },

Any ideas?