Fixed a bug that caused an exception if DLLs that failed to load were mixed in

[takahiro0327]

Fixed a bug that caused an exception if DLLs that failed to load were mixed in.
The IL2CPP version was circumvented by the is operation.
Merge if it looks good.

NullReferenceException: Object reference not set to an instance of an object
  at ConfigurationManager.SettingSearcher.CollectSettings (IEnumerable`1& results, System.Collections.Generic.List`1& modsWithoutSettings, Boolean showDebug) [0x00000] in <filename unknown>:0 
  at ConfigurationManager.ConfigurationManager.BuildSettingList () [0x00000] in <filename unknown>:0 
  at ConfigurationManager.ConfigurationManager.set_DisplayingWindow (Boolean value) [0x00000] in <filename unknown>:0 
  at ConfigurationManagerWrapper.ConfigurationManagerWrapper.Update () [0x00000] in <filename unknown>:0 
 
(Filename:  Line: -1)

Occurred by installing KoikatsuSunshine DLL (KKS_AssetImport.dll) on Koikatu (KK).

An exception during list generation prevents the plugin list in ConfigurationManager from being generated.