Android : Android unable to instantiate activity componentinfo classnotfoundexception

I've developed an app but when I run it, it crashes and shows the error: android unable to instantiate activity componentinfo classnotfoundexception.

I've read a lot about this error. I've tried Project -> Properties -> Java Build Path -> Order & Export and ensure Android Private Libraries and all other library projects are checked.

I've also noticed that when I press ctrl and pass the mouse over, for example, .MainActivity in my Android Manifest file, it doesn't display or link me to the Activity, don't know why.

Any ideas about this problem? Thank you so much.