NullReferenceException in compiler

[majocha]

Severity is probably low.

I noticed locally Error 193 in some tests that were possibly crashing the testhost in CI.
Running in debug I managed to capture a stacktrace so I post it here to not lose it:

 Interop.Required and init-only properties.F# should produce compile-time error when required properties are not specified in the initializer
   Source: RequiredAndInitOnlyProperties.fs line 295

  Iteration 75)   Interop.Required and init-only properties.F# should produce compile-time error when required properties are not specified in the initializer 
     Duration: 4 sec

    Message: 
System.AggregateException : One or more errors occurred. (Method Debug.Fail failed with 'Unknown exception seen in compiler: Microsoft.VisualStudio.TestPlatform.TestHost.DebugAssertException: Method Debug.Fail failed with 'Unknown exception seen in compiler: Microsoft.VisualStudio.TestPlatform.TestHost.DebugAssertException: Method Debug.Fail failed with 'Unknown exception seen in compiler: Microsoft.VisualStudio.TestPlatform.TestHost.DebugAssertException: Method Debug.Fail failed with 'Unknown exception seen in compiler: Microsoft.VisualStudio.TestPlatform.TestHost.DebugAssertException: Method Debug.Fail failed with 'Unknown exception seen in compiler: Microsoft.VisualStudio.TestPlatform.TestHost.DebugAssertException: Method Debug.Fail failed with 'Unknown exception seen in compiler: Microsoft.VisualStudio.TestPlatform.TestHost.DebugAssertException: Method Debug.Fail failed with 'Unknown exception seen in compiler: Microsoft.VisualStudio.TestPlatform.TestHost.DebugAssertException: Method Debug.Fail failed with 'Unknown exception seen in compiler: Microsoft.VisualStudio.TestPlatform.TestHost.DebugAssertException: Method Debug.Fail failed with 'Unexpected exception raised in compiler: Object reference not set to an instance of an object.
System.NullReferenceException: Object reference not set to an instance of an object.
   at FSharp.Compiler.Text.Layout.get_Tag()
   at FSharp.Compiler.Text.LayoutModule.isEmptyL(Layout layout) in E:\repos\fsharp\src\Compiler\Utilities\sformat.fs:line 295
   at FSharp.Compiler.Text.LayoutModule.mkNode(Layout l, Layout r, Joint joint) in E:\repos\fsharp\src\Compiler\Utilities\sformat.fs:line 309
   at FSharp.Compiler.Text.LayoutModule.op_HatHat(Layout layout1, Layout layout2) in E:\repos\fsharp\src\Compiler\Utilities\sformat.fs:line 322
   at FSharp.Compiler.NicePrint.InfoMemberPrinting.prettyLayoutOfPropInfoFreeStyle(TcGlobals g, ImportMap amap, Range m, DisplayEnv denv, PropInfo pinfo) in E:\repos\fsharp\src\Compiler\Checking\NicePrint.fs:line 1793
   at FSharp.Compiler.NicePrint.InfoMemberPrinting.formatPropInfoToBufferFreeStyle(TcGlobals g, ImportMap amap, Range m, DisplayEnv denv, StringBuilder os, PropInfo pinfo) in E:\repos\fsharp\src\Compiler\Checking\NicePrint.fs:line 1805
   at [email protected](StringBuilder buf) in E:\repos\fsharp\src\Compiler\Checking\NicePrint.fs:line 2805
   at Internal.Utilities.Library.Extras.buildString(FSharpFunc`2 f) in E:\repos\fsharp\src\Compiler\Utilities\lib.fs:line 286
   at FSharp.Compiler.NicePrint.Pipe #1 stage #1 at line [email protected](PropInfo pinfo)
   at FSharp.Compiler.NicePrint.Pipe #1 stage #1 at line [email protected](PropInfo x)
   at Microsoft.FSharp.Primitives.Basics.List.map[T,TResult](FSharpFunc`2 mapping, FSharpList`1 x) in E:\repos\fsharp\src\FSharp.Core\local.fs:line 249
   at Microsoft.FSharp.Collections.ListModule.Map[T,TResult](FSharpFunc`2 mapping, FSharpList`1 list) in E:\repos\fsharp\src\FSharp.Core\list.fs:line 98
   at FSharp.Compiler.NicePrint.multiLineStringOfPropInfos(TcGlobals g, ImportMap amap, Range m, DisplayEnv denv, FSharpList`1 pinfos) in E:\repos\fsharp\src\Compiler\Checking\NicePrint.fs:line 2810
   at FSharp.Compiler.CheckExpressions.CheckRequiredProperties[a](TcGlobals g, TcEnv env, TcFileState cenv, MethInfo minfo, FSharpList`1 finalAssignedItemSetters, Range mMethExpr) in E:\repos\fsharp\src\Compiler\Checking\Expressions\CheckExpressions.fs:line 1326
...

Full stack trace:
F# should produce compile-time error when required properties are not specified in the initializer-638719431092933566.testlog.txt

Repro, as usual, is to select a whole bunch of tests (for example Interop) in VS and "run until failure".