Fix long press crash.

[crazyhitty]

Don't execute longClickListener if clickableSpanUnderTouch is null. This would prevent the crash when no ClickableSpan is found under the touched location.

Fixes #9

[luongvo]

@Saketme It should be merged soon 👍

[saket]

This change avoids the listener from getting called, but doesn't solve the actual cause of the issue -- the long-press timer listener shouldn't get scheduled in the first place if a long-press isn't made on a clickable span. I will make the necessary changes. Thanks for the idea, @crazyhitty.

[crazyhitty]

Awesome :+1: