fix: check if today_button is defined before use it

src/index.js

@@ -540,8 +540,10 @@ export default class Gantt {
             'px';
 
         // Update the left value on page resize
+        if (this.$today_button) {
             this.$today_button.style.left = `${containerRect.left + 20}px`;
         }
+    }
 
     make_grid_ticks() {
         if (!['both', 'vertical', 'horizontal'].includes(this.options.lines))