doesn't work when using stack navigator (react navigation): null is not an object

When going back to the previous screen the app is crashing and I got this error: TypeError: null is not an object (evaluating 'canvas.getContext').

Otherwise its drawing on the proper screen.

[kasback]

Hello i think this should fix your problem :

if (canvas !== null){
            const ctx = canvas.getContext('2d');
            canvas.width = 500;
            canvas.height = 500;
            ctx.beginPath();
            ctx.arc(170, 150, 100, 0, 2 * Math.PI);
           ctx.stroke();
}

[KFernandoDilepix]

@kasback Indeed its working ! Thank you guys @iddan