fix: check if today_button is defined before use it

[thomasync]

No description provided.

[safwansamsudeen]

Thanks!

[mantou568]

this.$today_button.style.left = ${t.left + 20}px``

There is also a lack of judgment here, otherwise when today_futton is false, if the button does not exist here, an error will be reported

[safwansamsudeen]

@mantou568 this is the code:

if (this.$today_button) {
    this.$today_button.style.left = `${containerRect.left + 20}px`;
}