Steps to reproduce:
1. Create new project
2. Open Test Runner (Window -> Test Runner)
3. Create Test Assembly Folder
4. Create script in Tests folder
5. If script wasn't created from Test Runner window open it in Visual Studio

Expected results: No errors are printed in the Console window
Actual results: Two errors are printer om the Console window

Reproduced with: 2018.1.0b6, 2018.1.0b3
Not reproducible with: 2018.1.0b2, 2017.3.1f1
Regression introduced in: 2018.1.0b3

Error one:
TreeView root item is null. Ensure that your TreeViewDataSource sets up at least a root item.
UnityEditor.TestTools.TestRunner.TestRunnerWindow:CompilationCallback()

Error two:
InvalidOperationException: Operation is not valid due to the current state of the object
System.Linq.Enumerable.Single[ScriptAssembly] (IEnumerable`1 source, System.Func`2 predicate, Fallback fallback)
System.Linq.Enumerable.SingleOrDefault[ScriptAssembly] (IEnumerable`1 source, System.Func`2 predicate)
UnityEditor.TestTools.TestRunner.TestInEditorTestAssemblyProvider.AssemblySelector (System.String assemblyPath) (at C:/buildslave/unity/build/Extensions/TestRunner/UnityEditor.TestRunner/TestRunner/Utils/TestInEditorTestAssemblyProvider.cs:39)
System.Linq.Enumerable+<CreateWhereIterator>c__Iterator1D`1[System.String].MoveNext ()
System.Linq.Enumerable+<CreateSelectIterator>c__Iterator10`2[System.String,System.String].MoveNext ()
System.Collections.Generic.List`1[System.String].AddEnumerable (IEnumerable`1 enumerable) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections.Generic/List.cs:128)
System.Collections.Generic.List`1[System.String]..ctor (IEnumerable`1 collection) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections.Generic/List.cs:65)
System.Linq.Enumerable.ToList[String] (IEnumerable`1 source)
UnityEngine.TestTools.Utils.TestAssemblyProviderHelper.GetUserAssemblies (System.Func`2 assemblySelector, System.Func`2 selector) (at C:/buildslave/unity/build/Extensions/TestRunner/UnityEngine.TestRunner/Utils/TestAssemblyProviderHelper.cs:43)
UnityEditor.TestTools.TestRunner.TestInEditorTestAssemblyProvider.GetUserAssemblies () (at C:/buildslave/unity/build/Extensions/TestRunner/UnityEditor.TestRunner/TestRunner/Utils/TestInEditorTestAssemblyProvider.cs:31)
UnityEditor.TestTools.TestRunner.TestInEditorTestAssemblyProvider.GetTestsWithNUnit () (at C:/buildslave/unity/build/Extensions/TestRunner/UnityEditor.TestRunner/TestRunner/Utils/TestInEditorTestAssemblyProvider.cs:24)
UnityEditor.TestTools.TestRunner.GUI.EditModeTestListGUI.GetTestListNUnit () (at C:/buildslave/unity/build/Extensions/TestRunner/UnityEditor.TestRunner/GUI/Views/EditModeTestListGUI.cs:63)
UnityEditor.TestTools.TestRunner.GUI.TestListTreeViewDataSource.FetchData () (at C:/buildslave/unity/build/Extensions/TestRunner/UnityEditor.TestRunner/GUI/TestListTreeView/TestListTreeViewDataSource.cs:27)
UnityEditor.IMGUI.Controls.TreeViewDataSource.ReloadData () (at C:/buildslave/unity/build/Editor/Mono/GUI/TreeView/TreeViewDataSource.cs:53)
UnityEditor.IMGUI.Controls.TreeViewController.ReloadData () (at C:/buildslave/unity/build/Editor/Mono/GUI/TreeView/TreeViewController.cs:267)
UnityEditor.TestTools.TestRunner.GUI.TestListGUI.Reload () (at C:/buildslave/unity/build/Extensions/TestRunner/UnityEditor.TestRunner/GUI/Views/TestListGUIBase.cs:150)
UnityEditor.TestTools.TestRunner.TestRunnerWindow.OnEnable () (at C:/buildslave/unity/build/Extensions/TestRunner/UnityEditor.TestRunner/TestRunnerWindow.cs:115)