I had the same problem (adding an Oracle database project to my solution).
To fix the problem, you must edit the solution file (.sln). Find the Oracle Database project (find oradbproj). You should find something like this:
Project("{218574D1-FF94-4B95-8577-A6D58C11C315}") = "MyOracleDatabase", "MyOracleDatabase\MyOracleDatabase.oradbproj", "{41823BBF-36F6-42AC-9C41-119241BAAFEC}" EndProject
Later in the same .sln file, find the beginning of the section "GlobalSection (ProjectConfigurationPlatforms) = postSolution" . There would be many entries like these:
{CE85B8BD-1BCF-468C-AA0B-B869B87A66D5}.Debug|Any CPU.ActiveCfg = Debug|Any CPU {CE85B8BD-1BCF-468C-AA0B-B869B87A66D5}.Debug|Any CPU.Build.0 = Debug|Any CPU {CE85B8BD-1BCF-468C-AA0B-B869B87A66D5}.Release|Any CPU.ActiveCfg = Release|Any CPU {CE85B8BD-1BCF-468C-AA0B-B869B87A66D5}.Release|Any CPU.Build.0 = Release|Any CPU
Copy them by replacing the GUID with the second in the oradbproj line of the previous line ({41823BBF-36F6-42AC-9C41-119241BAAFEC} in the previous example):
{CE85B8BD-1BCF-468C-AA0B-B869B87A66D5}.Debug|Any CPU.ActiveCfg = Debug|Any CPU {CE85B8BD-1BCF-468C-AA0B-B869B87A66D5}.Debug|Any CPU.Build.0 = Debug|Any CPU {CE85B8BD-1BCF-468C-AA0B-B869B87A66D5}.Release|Any CPU.ActiveCfg = Release|Any CPU {CE85B8BD-1BCF-468C-AA0B-B869B87A66D5}.Release|Any CPU.Build.0 = Release|Any CPU {41823BBF-36F6-42AC-9C41-119241BAAFEC}.Debug|Any CPU.ActiveCfg = Debug|Any CPU {41823BBF-36F6-42AC-9C41-119241BAAFEC}.Debug|Any CPU.Build.0 = Debug|Any CPU {41823BBF-36F6-42AC-9C41-119241BAAFEC}.Release|Any CPU.ActiveCfg = Release|Any CPU {41823BBF-36F6-42AC-9C41-119241BAAFEC}.Release|Any CPU.Build.0 = Release|Any CPU
Now you can save the .sln file. If you edited the .sln file outside of Visual Studio, opening it, returning to Visual Studio, it will ask you to restart the solution. Click "Yes" and wait. I.e!!!
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