An error appearing on touchscreen Chromebooks

[dkhomenko-ioeducation]

We are getting an error that we cannot reproduce on our devices but we often see it in our logs:

Browser: Mozilla/5.0 (X11; CrOS x86_64 8743.85.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.101 Safari/537.36
JavaScript: Uncaught TypeError: Cannot read property '1' of null (Line: 1) Column: 7179 Stack Trace: TypeError: Cannot read property '1' of null
at CanvasRenderingContext2D.window.CanvasRenderingContext2D.CanvasRenderingContext2D.fillArea (/wPaint/wPaint.menu.main.custom.min.js:1:7179)
at Paint._drawBucketDown (/wPaint/wPaint.menu.main.custom.min.js:1:6441)
at Paint._callShapeFunc (/wPaint/wPaint.custom-0.0.2.js:394:42)
at HTMLCanvasElement.canvasMousedown (/wPaint/wPaint.custom-0.0.2.js:61:38)

According to the logs the actual piece of code where it's throwing an exception is this one (in wPaint.menu.main.min.js):
c = this.canvas.style.color.match(/^rgba?((.*));?$/)[1].split(",")

We already tried several different approaches to reproduce it but still cannot find what's causing this. The reason why the scripts in the stack trace above have "custom" in their names is because we changed the relative paths to images inside of the scripts. It's been working on all other devices so far but now is causing errors, but only on touchscreen Chromebooks and not every time, so we cannot actually reproduce it. Any idea why this can be happening?