Unfortunately I am unable to tell what exactly caused this issue. I do however quiet often get the following output from BuildOnSave using Visual Studio 2017:

---------- BuildOnSave CRASHED, please report to https://www.github.com/pragmatrix/BuildOnSave/issues
System.NullReferenceException: Object reference not set to an instance of an object.
at BuildOnSave.Projects.DependentProjectKeys(BuildDependencies dependencies, Project project)
at BuildOnSave.Projects.DependentMap(BuildDependencies dependencies, Project[] all)
at BuildOnSave.Projects.AffectedProjects(BuildDependencies dependencies, Project[] all, Project[] roots)
at BuildOnSave.BackgroundBuild2.tryMakeBuildRequest(String startupProject_, String[] changedProjectPaths)
at BuildOnSave.Driver.beginBuild(String startupProject_, String[] changedProjects)
at BuildOnSave.Driver.beginBuild(Solution solution, BuildType buildType, IEnumerable`1 projects)
at BuildOnSave.Driver.beginBuild()