How to reproduce:
1. Open the attached project 'unity.zip'
2. Select BurtCommon.dll in the Project window, make sure the Inspector tab is open
3. Tick the Linux x86_64 Platform in the Platform settings (un-tick it if it was ticked in the first place)
4. Click 'Apply'

Expected results: Linux x86_64 platform gets enabled
Actual results: Linux x64_64 is still disabled, NullReferenceException in the console

Reproducible with: 2019.1.9f1, 2019.2.0b9, 2019.3.0a5
Reproduced on CentOS which can't run versions past 2019.3.0a5.

Full error:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Experimental.AssetImporters.AssetImporterEditor.InternalSetTargets (UnityEngine.Object[] t) (at /home/builduser/buildslave/unity/build/Modules/AssetPipelineEditor/ImportSettings/AssetImporterEditor.cs:122)
UnityEditor.Editor.OnForceReloadInspector () (at /home/builduser/buildslave/unity/build/Editor/Mono/Inspector/Editor.cs:603)

Note: In 2019.3.0a1 instead of the NullReferenceException there are these errors:
Assertion failed on expression: 'targetHash.IsValid()'
Assertion failed on expression: 'loadedImportedAsset.targetHash.IsValid()'

Note 2: In 2019.2.0a1 and in 2019.1.9f1 it seems to work but there is another error in the console:
Win32Exception: ApplicationName='/run/media/pauliusg/Storage/Unities/2019.2.0a1/Editor/Data/Tools/RoslynScripts/unity_csc.sh', CommandLine='/noconfig @Temp/UnityTempFile-b69cd5a42947ef0768bc53765acb3b33', CurrentDirectory='', Native error= Out of memory

Note 3: Could not test in 2019.1.0a1 and in earlier version due to Platform settings not existing.