6.3.0 Beta bug - error occurs when installing Extensions

This is from a completely fresh install.

When trying to Install extensions, it downloads just fine then displays a message saying Error encountered. Details and the Details link shows this message:

An instance of task failed successfully perhaps?!

When you then refresh the page, you are able to simply press on Activate and the plugin is activated correctly.

This occurs with third party extensions (e.g. ACF) as well as Themeco Extensions (e.g. Custom 404)

Related to this, when now trying to delete any plugin, it shows this error message:

And hangs on saying Deleting...

Upon refresh, the plugin appears to have been deleted successfully.

I have a feeling this was related to the following forum post with extra lines at the start of some new PHP files. Let me know if you have this issue in Beta 2 and have a great day.