Null reference exception occurs on changing search location filter when collaborate is on and services window is active

Steps to Repro:
1. Create a new project
2. Window > general
3. Open services window and enable collab
4. Project window > search using a name
5. Change search location filter few times

Actual result:
1. Null reference exception occurs
2. Frequency rate is 2 out of 5 times

Occurring with:
2018.3.0a2, 2018.3.0a1

Not occurring with:
2018.2.0b9

Environment:
Only on Windows

Stack Trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEngine.GUISkin.GetStyle (System.String styleName) (at C:/buildslave/unity/build/Modules/IMGUI/GUISkin.cs:312)
UnityEngine.GUIStyle.op_Implicit (System.String str) (at C:/buildslave/unity/build/Modules/IMGUI/GUIStyle.cs:443)
UnityEditor.ProjectBrowser.InitSearchMenu () (at C:/buildslave/unity/build/Editor/Mono/ProjectBrowser.cs:528)
UnityEditor.ProjectBrowser.SetSearchViewState (SearchViewState state) (at C:/buildslave/unity/build/Editor/Mono/ProjectBrowser.cs:489)
UnityEditor.ProjectBrowser.SearchButtonClickedCallback (UnityEditor.ItemData itemClicked) (at C:/buildslave/unity/build/Editor/Mono/ProjectBrowser.cs:510)
UnityEditor.ExposablePopupMenu.SelectionChanged (UnityEditor.ItemData item) (at C:/buildslave/unity/build/Editor/Mono/GUI/ExposablePopupMenu.cs:125)
UnityEditor.ExposablePopupMenu+PopUpMenu.SelectionCallback (System.Object userData) (at C:/buildslave/unity/build/Editor/Mono/GUI/ExposablePopupMenu.cs:153)
UnityEditor.GenericMenu.CatchMenu (System.Object userData, System.String[] options, Int32 selected) (at C:/buildslave/unity/build/Editor/Mono/GUI/GenericMenu.cs:119)