🐛 Add guard when calling tokenLinesForScreenRows

It should prevent #481 from happening

lib/mixins/canvas-drawer.js

@@ -482,11 +482,17 @@ export default class CanvasDrawer extends Mixin {
     const context = this.tokensLayer.context
     const {width: canvasWidth} = this.tokensLayer.getSize()
 
+    if (!this.tokenLinesForScreenRows) { return }
+
+    const screenRowsTokens = this.tokenLinesForScreenRows(firstRow, lastRow)
+
     let y = offsetRow * lineHeight
-    for (let tokens of this.tokenLinesForScreenRows(firstRow, lastRow)) {
+    for (let i = 0; i < screenRowsTokens.length; i++) {
+      let tokens = screenRowsTokens[i]
       let x = 0
       context.clearRect(x, y, canvasWidth, lineHeight)
-      for (let token of tokens) {
+      for (let j = 0; j < tokens.length; j++) {
+        let token = tokens[j]
         if (/^\s+$/.test(token.value)) {
           x += token.value.length * charWidth
         } else {