foreach (DataRelation rs in dbComm.ds.Relations)
{
    Debug.WriteLine(rs.RelationName);
    Debug.WriteLine(rs.ParentKeyConstraint.ConstraintName);
    Debug.WriteLine(rs.ChildKeyConstraint.ConstraintName);
    foreach (DataColumn col in rs.ChildColumns)
    {
        Debug.WriteLine(col.ColumnName);
    }
    foreach (DataColumn col in rs.ParentColumns)
    {
        Debug.WriteLine(col.ColumnName);
    }
}

for (int i = 0; i < this.ultraGanttView1.Project.Tasks.Count; i++)
{
    if(this.ultraGanttView1.Project.Key == this.ultraGanttView1.Project.Tasks[i].ProjectKey)
    {
        Debug.WriteLine("SAME");
    } else
    {
        Debug.WriteLine("NOT THE SAME");
    }
}

UltraPrintPreviewDialog printPreview = new UltraPrintPreviewDialog();
printPreview.Document = this.ultraGanttViewPrintDocument1;
this.ultraGanttViewPrintDocument1.GanttView = this.ultraGanttView1;
printPreview.ShowDialog(this);//The error comes on this line

Error that I get:

Output that I get in the debugger:

Relation1
Constraint1
Relation1
ProjectKey
ProjectKey
SAME
SAME
'ProjectPlanning.exe' (CLR v4.0.30319: ProjectPlanning.exe): Loaded 'C:\Windows\Microsoft.Net\assembly\GAC_MSIL\Infragistics4.Win.UltraWinStatusBar.v17.2\v4.0_17.2.20172.2006__7dd5c3163f2cd0cb\Infragistics4.Win.UltraWinStatusBar.v17.2.dll'. Skipped loading symbols. Module is optimized and the debugger option 'Just My Code' is enabled.
Exception thrown: 'System.Data.InvalidConstraintException' in System.Data.dll
An exception of type 'System.Data.InvalidConstraintException' occurred in System.Data.dll but was not handled in user code
ForeignKeyConstraint Relation1 requires the child key values () to exist in the parent table.

Thanks in advance,

Robbe