Callback for makeImageData function not called!

GoJS
1

I am using sample below to download diagram as image file, using makeImageData function

Due to some reason, the callback function is never called. Can you tell me possible reason for this ? I am using GoJS v1.6.18

2

I believe this is what changed in later version(s)

1.7.4 => Fixed broken Diagram.makeImageData for “blob” return type.