Error message when I click the "add" button in the operations tab

I get the following error message when I try to add a new operation to the properties of a class:

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

   at SoftwareIdeasModeler.Dialogs.Properties.Fields.OperationEditorForm.BindData()

   at SoftwareIdeasModeler.Dialogs.Properties.Fields.OperationEditorForm..ctor(Project project, OperationField operation)

   at SoftwareIdeasModeler.Dialogs.Properties.Fields.OperationsControl.addOperationButton_Click(Object sender, EventArgs e)

   at System.Windows.Forms.Control.OnClick(EventArgs e)

   at System.Windows.Forms.Button.OnClick(EventArgs e)

   at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)

   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)

   at System.Windows.Forms.Control.WndProc(Message& m)

   at System.Windows.Forms.ButtonBase.WndProc(Message& m)

   at System.Windows.Forms.Button.WndProc(Message& m)

   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

OF=SoftwareIdeasModeler.MainForm

LastCmd=SoftwareIdeasModeler.Core.Commands.Layout.MoveCommand

OF=SoftwareIdeasModeler.Controls.TabSwitch.ThumbnailWindow

OF=SoftwareIdeasModeler.Dialogs.Properties.ItemPropertiesForm