An error occurs when trying to schedule activities in Microsoft Dynamics CRM: "An unhandled exception occurred"

After removing either the Site or Customers fields from the Service Activity form, the following error will occur when trying to schedule activities:

Microsoft CRM Error Report:
Error Description:
An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Error Details:
Exception of type 'System.Web.HttpUnhandledException' was thrown.

Full Stack:
[KeyNotFoundException: The given key was not present in the dictionary.]
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at Microsoft.Crm.Application.Dialogs.SM.ActivitySchedulingDialog.SetCustomizableLabel(FormDescriptor formDescriptor, EntityMetadata serviceActivityMetadata, String attributeName, FormCell formCell)
   at Microsoft.Crm.Application.Dialogs.SM.ActivitySchedulingDialog.SetCustomizableLabels()
   at Microsoft.Crm.Application.Dialogs.SM.ActivitySchedulingDialog.ConfigureForm()
   at Microsoft.Crm.Application.Controls.AppUIPage.OnPreRender(EventArgs e)
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

[HttpUnhandledException: Exception of type 'System.Web.HttpUnhandledException' was thrown.]
   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)