How to reproduce:
1. Download and open attached "NavMesh_Bug2" project
2. Change Layout of Editor
3. Observe Console and enter Play Mode

Expected result: Console is empty of errors and Editor enters Play Mode
Actual result: Console is filled with errors and Editor does not enter Play Mode

Reproduced with: 2018.3.0b1

Main error:
NullReferenceException: Object reference not set to an instance of an object
at System.Text.RegularExpressions.FactoryCache+Key.Equals (System.Object o) [0x00000] in <filename unknown>:0
at System.Text.RegularExpressions.MRUList.Use (System.Object o) [0x00000] in <filename unknown>:0
at System.Text.RegularExpressions.FactoryCache.Add (System.String pattern, RegexOptions options, IMachineFactory factory) [0x00000] in <filename unknown>:0
at System.Text.RegularExpressions.Regex.InitNewRegex () [0x00000] in <filename unknown>:0
at System.Text.RegularExpressions.Regex.Init () [0x00000] in <filename unknown>:0
at System.Text.RegularExpressions.Regex..ctor (System.String pattern, RegexOptions options) [0x00000] in <filename unknown>:0
at UnityEditor.Scripting.Compilers.MonoCSharpCompilerOutputParser..cctor () [0x00000] in C:\buildslave\unity\build\Editor\Mono\Scripting\Compilers\MonoCSharpCompilerOutputParser.cs:7

Notes: Reopening project fixes the issue.