Check if this.$el is defined in isAllItemsInCurrentPageSelected (issue 619)

[ramonakira]

PR belonging to issue #619

[SparoHawk]

While this does fix the rendering issue, it breaks the functionality of the checkbox in the header row. After hiding and showing the field you can no longer use it to select all fields. Also, selecting all fields will not select the header checkbox.

Example: https://codesandbox.io/s/d48fc3f36b21-8s0cj