Patch for Canvas getting nulled by garbage collection

[masterX244]

Another patch found in the forums that might be useful for developers:
https://www.sencha.com/forum/showthread.php?302549-Ext-5.1.0.107-Chart-canvas-destroyed-leads-to-empty-chart
It fixes a issue when a chart is created but not visible immediately that the canvas gets nulled out and when trying to render the chart is fails with a weird glitched rendering

[themightychris]

Hi @masterX244, thanks for sharing!

Would you have some time to open a pull request against the appropriate branch? See this old PR for some comments on how to best format it: #3 (comment)