[NuGet] Fix exception when changing the target framework of a project.

Fixed bug 23285 - NRE when changing the target framework of an Android
project.
https://bugzilla.xamarin.com/show_bug.cgi?id=23285

The null reference exception would occur when doing the following:

1) Create a project with one NuGet package added.
2) Create two NuGet package sources in Preferences. Check both these
two sources so they are both enabled. All the rest should be disabled.
3) Open the Add Packages dialog and select select All Sources.
4) Go back to Preferences and uncheck both of the two package sources.
5) Change the target framework of the project in Build/General.

Then you see an error in the Package Console window:

 Object reference not set to an instance of an object.

And the following error in the IDE log:

System.NullReferenceException: Object reference not set to an instance
of an object.
PackageSourceExtensions.IsAggregate(PackageSource packageSource)
RegisteredPackageRepositories.CreateActiveRepository()
RegisteredPackageRepositories.get_ActiveRepository()
PackageCompatibilityChecker.CheckProjectPackages(IDotNetProject project)
PackageCompatibilityRunner.CheckCompatibility()