Test runner errors InvalidOperationException and TreeView root item is null

Hi

I'm trying to use tests in my project. I'm working on newest Unity version - 2017.2.0f3. I've encountered some errors that I have no idea how to resolve. If someone could help me it would be really great. Basically every time I open Test Runner I get two errors:

 InvalidOperationException: Operation is not valid due to the current state of the object
 System.Linq.Enumerable.Single[PrecompiledAssembly] (IEnumerable`1 source, System.Func`2 predicate, Fallback fallback)
 System.Linq.Enumerable.SingleOrDefault[PrecompiledAssembly] (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:41)
 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:45)
 UnityEditor.TestTools.TestRunner.TestInEditorTestAssemblyProvider.GetUserAssemblies () (at C:/buildslave/unity/build/Extensions/TestRunner/UnityEditor.TestRunner/TestRunner/Utils/TestInEditorTestAssemblyProvider.cs:30)
 UnityEditor.TestTools.TestRunner.TestInEditorTestAssemblyProvider.GetTestsWithNUnit () (at C:/buildslave/unity/build/Extensions/TestRunner/UnityEditor.TestRunner/TestRunner/Utils/TestInEditorTestAssemblyProvider.cs:23)
 UnityEditor.TestTools.TestRunner.GUI.PlayModeTestListGUI.GetTestListNUnit () (at C:/buildslave/unity/build/Extensions/TestRunner/UnityEditor.TestRunner/GUI/Views/PlayModeTestListGUI.cs:86)
 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:266)
 UnityEditor.TestTools.TestRunner.GUI.TestListGUI.Reload () (at C:/buildslave/unity/build/Extensions/TestRunner/UnityEditor.TestRunner/GUI/Views/TestListGUIBase.cs:147)
 UnityEditor.TestTools.TestRunner.TestRunnerWindow.OnEnable () (at C:/buildslave/unity/build/Extensions/TestRunner/UnityEditor.TestRunner/TestRunnerWindow.cs:113)

and the second one:

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

I get these errors with or without any written tests in project. For the record the project is new and clean, I'm using some third party libraries: Entitas, FMOD, Firebase and Crashlytics.

If you can help me in any way I will really appreciate it, thanks!,