Nuget error "Object reference not set to an instance of an object" while installing Entityframework 7 from Package Manager Console in VS2015

I had a similar issue in Visual Studio 2015 Community Edition.

Created a simple ASP.NET MVC 5 default project that I hosted on Azure. I was working through the project and when I tried to load up the NuGet GUI but kept getting the "Object Reference..." error.

I restarted Visual Studio in Administrator mode (right-clicked Visual Studio and started in Administrator mode) which resolved the problem.

I have solved this problem, by disabling the "https://api.nuget.org/v3/" from the package source, in order to do this, Choose Tools > Nuget Package Manager > Package Source

And other thing I did is added myget package source "https://www.myget.org/F/aspnetvnext/api/v2/"

Following the steps Choose Tools > Nuget Package Manager > Package Source > Click add on top of the window and add the myget package source.

I solved my problem by adding:

https://api.nuget.org/v3/index.json

as a Package Sources feed, as shon on the picture below: