Bug #47958 Error when closing window

Description:
Error when closing Relationship Window on ERR Diagram.

Error message:
System.NullReferenceException: Referência de objeto não definida para uma instância de um objeto.
   em MySQL.GUI.Workbench.MainForm.mainDockPanel_ActiveDocumentClosing(Object sender, ActiveDocumentClosingEventArgs e)
   em WeifenLuo.WinFormsUI.Docking.DockPanel.OnActiveDocumentClosing(ActiveDocumentClosingEventArgs e)
   em WeifenLuo.WinFormsUI.Docking.DockPane.CloseContent(IDockContent content)
   em WeifenLuo.WinFormsUI.Docking.DockPane.CloseActiveContent()
   em WeifenLuo.WinFormsUI.Docking.VS2005DockPaneStrip.Close_Click(Object sender, EventArgs e)
   em System.Windows.Forms.Control.OnClick(EventArgs e)
   em System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   em System.Windows.Forms.Control.WndProc(Message& m)
   em System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   em System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   em System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

How to repeat:
- Create a ERR Diagram
- Create some tables and relationships
- Double click on "relationship line" to open a Relationship window
- Close the relationship window and get the error

Fixed in 5.2

Bug #48023 has been marked as duplicate of this one

this has been fixed in our repository and will be included in 5.2.5

An entry was added to the 5.2.5 changelog:

The following exception was generated when closing the Relationship tab for an EER Diagram:

System.NullReferenceException: Referência de objeto não definida para uma instância de
um objeto.
   em MySQL.GUI.Workbench.MainForm.mainDockPanel_ActiveDocumentClosing(Object sender,
ActiveDocumentClosingEventArgs e)
   em
WeifenLuo.WinFormsUI.Docking.DockPanel.OnActiveDocumentClosing(ActiveDocumentClosingEventA
rgs e)
   em WeifenLuo.WinFormsUI.Docking.DockPane.CloseContent(IDockContent content)
   em WeifenLuo.WinFormsUI.Docking.DockPane.CloseActiveContent()
   em WeifenLuo.WinFormsUI.Docking.VS2005DockPaneStrip.Close_Click(Object sender,
EventArgs e)
   em System.Windows.Forms.Control.OnClick(EventArgs e)
   em System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32
clicks)
   em System.Windows.Forms.Control.WndProc(Message& m)
   em System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   em System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   em System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam,
IntPtr lparam)