If you are having the two symptoms of the preview/output showing up as gray as well as ProPresenter crashing when you quit it, then most likely your hard drive is in "Case Sensitive" mode.
A bit of background: There are multiple ways of formatting a hard drive. On a Mac, the most common file format is "MacOS Extended (Journaled)" (also known as "HFS+"). In High Sierra Apple introduced a new file format called APFS (Apple File System). However, each of these file formats also include the option to be "Case Sensitive". ProPresenter (as well as many other professional level applications) is not supported in Case Sensitive mode, so if your hard drive is set to Case Sensitive then ProPresenter will have the aforementioned symptoms.
The only full way to fix this issue would be to reformat your hard drive, this time as non-Case Sensitive (that is with a file format type that does not list "Case Sensitive"). There are many great articles on the web that will walk you through this process, but know that you will lose ALL of the data on your computer when you do so, so creating a backup of your data will be very important.
There is a temporary workaround if you don't want to reformat your hard drive immediately. If you download ProPresenter from our website it will first be contained within a "disk image" (this is where it is asking you to drag it to the Application folder). This disk image is NOT Case Sensitive, so instead of dragging ProPresenter to your Application folder, simply run it from the disk image. Note that we don't recommend this as a long-term solution, but it will get ProPresenter up and running with no symptoms for the time being.