Fix test

Author: jeffkl

src/Microsoft.Build.Utilities.ProjectCreation.UnitTests/SolutionTests.cs

@@ -28,7 +28,7 @@ public void BasicTest()
 
             File.ReadAllText(solutionFileFullPath).ShouldBe(
                 @$"Microsoft Visual Studio Solution File, Format Version 12.00
-Project(""{{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}}"") = ""{project1Name}"", ""{project1FullPath}"", ""{{{projectInSolution.Id.ToString().ToUpperInvariant()}}}""
+Project(""{{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}}"") = ""{project1Name}"", ""{project1FullPath.Replace(Path.PathSeparator, '\\')}"", ""{{{projectInSolution.Id.ToString().ToUpperInvariant()}}}""
 EndProject
 Global
 	GlobalSection(SolutionProperties) = preSolution

src/Microsoft.Build.Utilities.ProjectCreation/SolutionCreator.cs

@@ -211,6 +211,11 @@ public SolutionCreator TryProject(ProjectCreator project, out SolutionProjectMod
         /// <returns>The current <see cref="SolutionCreator" />.</returns>
         public SolutionCreator TryProject(ProjectCreator project, SolutionFolderModel? folder, out SolutionProjectModel projectInSolution, string? projectTypeName = null)
         {
+            if (string.IsNullOrWhiteSpace(project.FullPath))
+            {
+                throw new InvalidOperationException("The project must have a valid path before it can be added to the solution.");
+            }
+
             project.Save();
 
             projectInSolution = _lastProject = _solutionModel.AddProject(project.FullPath, projectTypeName, folder);