Project change results in error with view only rights to a custom field.

The following error occurs when attempting to change the Project for a bug where the user has view only rights to at least one custom field:

Server Error in '/tracker' Application.

Object reference not set to an instance of an object.
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.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 2558: else
Line 2559: {
Line 2560: text = Convert.ToString(dr_bug[column_name]);
Line 2561: }
Line 2562:

Source File: c:\bugtracker\edit_bug.aspx Line: 2560

Stack Trace:

[NullReferenceException: Object reference not set to an instance of an object.]
ASP.edit_bug_aspx.display_custom_fields() in c:\bugtracker\edit_bug.aspx:2560
ASP.edit_bug_aspx.__Render__control2(HtmlTextWriter __w, Control parameterContainer) in c:\bugtracker\edit_bug.aspx:3107
System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +109
System.Web.UI.HtmlControls.HtmlForm.RenderChildren(HtmlTextWriter writer) +173
System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer) +31
System.Web.UI.HtmlControls.HtmlForm.Render(HtmlTextWriter output) +53
System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +100
System.Web.UI.HtmlControls.HtmlForm.RenderControl(HtmlTextWriter writer) +40
ASP.edit_bug_aspx.__Render__control1(HtmlTextWriter __w, Control parameterContainer) in c:\bugtracker\edit_bug.aspx:2951
System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +109
System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +8
System.Web.UI.Page.Render(HtmlTextWriter writer) +29
System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +100
System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3060

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.272