You may come across the following error when you try to open a project that was created using Developer toolkit
(These file records all of the options that you might associate with your solution so that each time you open it, it includes customizations that you have made)
This .SUO file is hidden in the project folder next to solution file which you can see in below screenshot.
Simply delete the above selected red rectangle file(.SUO) and run solution file.
Now it will prompt for Connect CRM Explorer.
Note: SUO (solution user options) are user settings files. Completely disposable, you can delete it and the most common thing you will lose are breakpoints.