Null reference exception when call dispose

In the dispose method of a Form I'm calling dispose to some controls to help GC to collect they, but when I dispose the dockmanager object i'm getting this exception:

Message: {"Referencia a objeto no establecida como instancia de un objeto."} (Null reference exception in spanish)

Source: "ActiproSoftware.UIStudio.Dock.Net20"
StackTrace:
en ActiproSoftware.UIStudio.Dock.DockManager.a(ContainerControl A_0)
en ActiproSoftware.UIStudio.Dock.DockManager.Dispose(Boolean disposing)
en System.ComponentModel.Component.Dispose()
en VSControls.Grupo.Dispose(Boolean disposing) en N:\Proyectos\Controles\Controles\Ventanas\Principal\Controles\Grupo.vb:línea 1906

I hope this helps you, thanks!