Add a check at the point that the PDF will be drawn

[sscholle]

Scenario:
User navigates away from page before PDF has loaded. The drawPDF callback triggers but the canvas element is no longer rendering.

Error:
TypeError: Cannot read property 'getContext' of null

Source:

react-pdf-js/src/index.tsx

Line 109 in 80c2025

const canvasContext = canvas.getContext('2d');

Proposed solution:
add a simple check before getContext is called.

[sscholle]

turns out my code is out of date....