When I convert stage canvas to png, sometimes the image is full with black

I use toDataUrl to convert stage canvas to png, but sometimes the image is all black and with nothing else. Miraculously, there are times when the image displays the contents of the canvas normally. Does anyone know why?

Here is my code:

const canvas = document.querySelector('.stage_stage_1fD7k');
const image = canvas.toDataUrl('image/png');
const link = document.createElement('a');
link.href = image;
link.download = 'balabala.png';
const event = document.createEvent('MouseEvents');
event.initMouseEvent('click', true, false, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null);
link.dispatchEvent(event);

[summerscar]

https://github.com/LLK/scratch-render/commit/550cd7aacf9a5df74d14af63cd5e8388dee6972f

[paulkaplan]

See https://github.com/LLK/scratch-gui/blob/ab31d36251e21fd78aac34983ede6a75e9f1ff71/src/lib/project-saver-hoc.jsx#L66 for the right way to capture stage images