Problem

If you:

1. Have exactly one game instance
2. Set at least one additional compatible version in the compatible versions dialog
3. Remove buildID.txt and readme.txt, without removing the KSP folder or the CKAN folder
4. Launch CKAN

... then on load, you get the compatible versions popup warning you about a version change (if in GUI), and clicking Save causes an exception:

************** Exception Text **************
System.NullReferenceException: Object reference not set to an instance of an object.
   at CKAN.KSP.SaveCompatibleVersions()
   at CKAN.CompatibleKspVersionsDialog.SaveButton_Click(Object sender, EventArgs e)
   at System.Windows.Forms.Control.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ButtonBase.WndProc(Message& m)
   at System.Windows.Forms.Button.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

(One way to make this happen is apparently to uninstall in Steam, then re-install to a different Steam library folder. See #2443.)

The problem is in Core, so the other UIs are also affected, but the details vary.

Causes

KSP.Version() is allowed to be null in most places (and it happens when the buildID.txt and readme.txt files are missing), but the code to load and save compatible versions doesn't like it. (This is somewhat ironic as that code would be one of the better ways of handling this situation, since you could use it to set the correct game version manually.)

For saving, ToString() is called on the null reference.

For loading, if compatible_ksp_versions.json has a null value for the VersionOfKspWhenWritten property, it is passed to KspVersion.Parse, which throws exceptions on null. (That property can't be null currently, but it will be possible once we allow the save to proceed with a null.)

To be considered invalid, a game instance must lack a GameData folder. If it merely lacks the files that provide the game version, this is treated as fine; you can attempt to open and use such an instance even though we know it's missing at least some data that should be there.

Changes

Now the null conditional operator ("?.") is used to avoid a null reference exception at save. This allows a null value to be written to the file.

Now we use KspVersion.TryParse instead of Parse to allow VersionOfKspWhenCompatibleVersionsWereStored to be set to null at load.

Finally, a game instance without a version is now treated as invalid. This means you can't open them from the Select KSP Install window, and they won't be auto-opened at launch. If your only known instance loses its version info (say, by uninstalling in Steam), then it'll automatically open the Select KSP Install window instead.

Fixes #2443.