当renderer设置为"canvas"时，graph.toDataURL()生成空白页 #3323

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Reproduction link

https://antv-g6.gitee.io/zh/examples/item/defaultNodes#circle

Steps to reproduce

将'renderer'设置为'canvas',然后在graph.render();之后添加'let dataurl = graph.toDataURL()',生成的url为空白页面

将'renderer'设置为'svg'时正常

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