When importing some projects rom the Asset Store that are a bit more scripting-heavy, this error apears int the console:

ArgumentException: Value does not fall within the expected range.
SyntaxTree.VisualStudio.Unity.Bridge.CompilationUnit.LanguageOf (SyntaxTree.VisualStudio.Unity.Bridge.CompilationUnit unit)
SyntaxTree.VisualStudio.Unity.Bridge.CompilationUnit.CompilationUnits (System.String method)
SyntaxTree.VisualStudio.Unity.Bridge.CompilationUnit.CompilationUnits ()
SyntaxTree.VisualStudio.Unity.Bridge.ProjectSystem.UnitySolutionBuilder..ctor ()
SyntaxTree.VisualStudio.Unity.Bridge.ProjectSystem.UnitySolutionBuilder.CreateSolutionFromAssetDatabase ()
SyntaxTree.VisualStudio.Unity.Bridge.ProjectFilesGenerator.GenerateProject ()
SyntaxTree.VisualStudio.Unity.Bridge.ProjectFilePostprocessor.OnPreGeneratingCSProjectFiles ()
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:232)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115)
UnityEditor.AssetPostprocessingInternal.OnPreGeneratingCSProjectFiles () (at C:/buildslave/unity/build/Editor/Mono/AssetPostprocessor.cs:61)
UnityEditor.VisualStudioIntegration.SolutionSynchronizer.Sync () (at C:/buildslave/unity/build/Editor/Mono/VisualStudioIntegration/SolutionSynchronizer.cs:198)
UnityEditor.VisualStudioIntegration.SolutionSynchronizer.SyncIfNeeded (IEnumerable`1 affectedFiles) (at C:/buildslave/unity/build/Editor/Mono/VisualStudioIntegration/SolutionSynchronizer.cs:187)
UnityEditor.SyncVS.PostprocessSyncProject (System.String[] importedAssets, System.String[] addedAssets, System.String[] deletedAssets, System.String[] movedAssets, System.String[] movedFromAssetPaths) (at C:/buildslave/unity/build/Editor/Mono/SyncProject.cs:177)
UnityEditor.AssetPostprocessingInternal.PostprocessAllAssets (System.String[] importedAssets, System.String[] addedAssets, System.String[] deletedAssets, System.String[] movedAssets, System.String[] movedFromPathAssets) (at C:/buildslave/unity/build/Editor/Mono/AssetPostprocessor.cs:31)

Repro steps:
Start a new project with Unity 2018.1
Import an asset store pack, like DoTween: http://u3d.as/aZ1
Notice the error
Double click a script to bring up Visual Studio
Notice there's no project in the solution

This issue does not appear in 2017.3 or any previous major releases or 2018.1 (tested a1->b1)