exportImage(), exportSvg() fails with "Export function called before canvas loaded"
https://github.com/vinothpandian/react-sketch-canvas/blob/bcbb1418a8f3d1cf6f99cacfb07d07487fa134c7/packages/react-sketch-canvas/src/ReactSketchCanvas/index.tsx#L50

`exportSvg()` sometimes throws with `Export function called before canvas loaded` because `svgCanvas.current` is `null` - this is due to the usage of `createRef` which creates a new ref every time the component is rendered. 

Suggested fix: replace `createRef` with `useRef`