Data Compare - Filtering

This is a portion of my code, where I try to filter out tables that I do not want to synch. I am getting an "Object reference not set to an instance of an object error" in the loop. So it did not finish the loop. I have about 440 tables to loop thru and it stop at 219.

Database datadb1 = new Database();
datadb1.RegisterForDataCompare(new ConnectionProperties(this.txtServer.Text.ToString(), tempDB, this.txtUser.Text.ToString(), this.txtPwd.Text.ToString()));

Database datadb2 = new Database();
datadb2.RegisterForDataCompare(new ConnectionProperties(this.txtServer.Text.ToString(), this.txtDB.Text.ToString(), this.txtUser.Text.ToString(), this.txtPwd.Text.ToString()));
RedGate.SQLDataCompare.Engine.SchemaMappings mappings = new RedGate.SQLDataCompare.Engine.SchemaMappings();

mappings.CreateMappings(datadb1, datadb2);

foreach (RedGate.SQLDataCompare.Engine.TableMapping mapping in mappings.TableMappings)
{
if (DataSynch.ToLower().IndexOf(";" + mapping.Obj1.Name.ToLower() + ";") < 0)
mapping.Include = false;
}

RedGate.SQLDataCompare.Engine.ComparisonSession session = new RedGate.SQLDataCompare.Engine.ComparisonSession();
session.Status += new StatusEventHandler(StatusCallback);

session.CompareDatabases(datadb1, datadb2, mappings);