External column picker error

Hi,

I try to set the external column, but I get error:

Application error when access /_layouts/15/Picker.aspx, Error=Value cannot be null.  Parameter name: typeName   at System.Type.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase)     at Microsoft.SharePoint.WebPartPages.Utility.GetTypeFromAssembly(String typeName, Boolean throwOnError, Boolean ignoreCase, Boolean safeLoad)     at Microsoft.SharePoint.ApplicationPages.Picker.OnLoad(EventArgs e)     at System.Web.UI.Control.LoadRecursive()     at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

System.ArgumentNullException: Value cannot be null.  Parameter name: typeName    at System.Type.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase)     at Microsoft.SharePoint.WebPartPages.Utility.GetTypeFromAssembly(String typeName, Boolean throwOnError, Boolean ignoreCase, Boolean safeLoad)     at Microsoft.SharePoint.ApplicationPages.Picker.OnLoad(EventArgs e)     at System.Web.UI.Control.LoadRecursive()     at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Getting Error Message for Exception System.Web.HttpUnhandledException (0x80004005): Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.ArgumentNullException: Value cannot be null.  Parameter name: typeName     at System.Type.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase)     at Microsoft.SharePoint.WebPartPages.Utility.GetTypeFromAssembly(String typeName, Boolean throwOnError, Boolean ignoreCase, Boolean safeLoad)     at Microsoft.SharePoint.ApplicationPages.Picker.OnLoad(EventArgs e)     at System.Web.UI.Control.LoadRecursive()     at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)     at System.Web.UI.Page.HandleError(Exception e)     at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)     at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)     at System.Web.UI.Page.ProcessRequest()     at System.Web.UI.Page.ProcessRequest(HttpContext context)     at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()     at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)A

Please help,

I don't understand, where is the problem.

Thanks,

Mykie