exportImage(), exportSvg() fails with "Export function called before canvas loaded"

[dhe128]

react-sketch-canvas/packages/react-sketch-canvas/src/ReactSketchCanvas/index.tsx

Line 50 in bcbb141

const svgCanvas = React.createRef<CanvasRef>();

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