Description of problem:
In the current version of gnutu some buttons are disabled and programm can not be used, no Russian, errors in Bulgarian, don't work with school subjects:

(gnutu:16332): Gtk-CRITICAL **: gtk_list_store_get_value: assertion
`VALID_ITER (iter, list_store)' failed

(gnutu:16332): GLib-GObject-CRITICAL **: g_value_unset: assertion
`G_IS_VALUE (value)' failed
Marshaling changed signal
Exception in Gtk# callback delegate
  Note: Applications can use GLib.ExceptionManager.UnhandledException
to handle the exception.
System.Reflection.TargetInvocationException: Exception has been thrown
by the target of an invocation. ---> System.NullReferenceException:
Object reference not set to an instance of an object
  at gnutu.GUI.EditLessonsWindow.on_Hour_changed (System.Object o,
System.EventArgs args) [0x00000] in <filename unknown>:0
  at (wrapper managed-to-native)
System.Reflection.MonoMethod:InternalInvoke
(System.Reflection.MonoMethod,object,object[],System.Exception&)
  at System.Reflection.MonoMethod.Invoke (System.Object obj,
BindingFlags invokeAttr, System.Reflection.Binder binder,
System.Object[] parameters, System.Globalization.CultureInfo culture)
[0x00000] in <filename unknown>:0
  --- End of inner exception stack trace ---
  at System.Reflection.MonoMethod.Invoke (System.Object obj,
BindingFlags invokeAttr, System.Reflection.Binder binder,
System.Object[] parameters, System.Globalization.CultureInfo culture)
[0x00000] in <filename unknown>:0
  at System.Reflection.MethodBase.Invoke (System.Object obj,
System.Object[] parameters) [0x00000] in <filename unknown>:0
  at System.Delegate.DynamicInvokeImpl (System.Object[] args)
[0x00000] in <filename unknown>:0
  at System.MulticastDelegate.DynamicInvokeImpl (System.Object[] args)
[0x00000] in <filename unknown>:0
  at System.Delegate.DynamicInvoke (System.Object[] args) [0x00000] in
<filename unknown>:0
  at GLib.Signal.ClosureInvokedCB (System.Object o,
GLib.ClosureInvokedArgs args) [0x00000] in <filename unknown>:0
  at GLib.SignalClosure.Invoke (GLib.ClosureInvokedArgs args)
[0x00000] in <filename unknown>:0
  at GLib.SignalClosure.MarshalCallback (IntPtr raw_closure, IntPtr
return_val, UInt32 n_param_vals, IntPtr param_values, IntPtr
invocation_hint, IntPtr marshal_data) [0x00000] in <filename
unknown>:0
   at GLib.ExceptionManager.RaiseUnhandledException(System.Exception
e, Boolean is_terminal)
   at GLib.SignalClosure.MarshalCallback(IntPtr raw_closure, IntPtr
return_val, UInt32 n_param_vals, IntPtr param_values, IntPtr
invocation_hint, IntPtr marshal_data)
   at Gtk.ComboBox.gtk_combo_box_remove_text(IntPtr , Int32 )
   at Gtk.ComboBox.RemoveText(Int32 position)
   at gnutu.GUI.EditLessonsWindow.on_AddBtn_clicked(System.Object o,
System.EventArgs args)
   at System.Reflection.MonoMethod.InternalInvoke(System.Reflection.MonoMethod
, System.Object , System.Object[] , System.Exception ByRef )
   at System.Reflection.MonoMethod.Invoke(System.Object obj,
BindingFlags invokeAttr, System.Reflection.Binder binder,
System.Object[] parameters, System.Globalization.CultureInfo culture)
   at System.Reflection.MethodBase.Invoke(System.Object obj,
System.Object[] parameters)
   at System.Delegate.DynamicInvokeImpl(System.Object[] args)
   at System.MulticastDelegate.DynamicInvokeImpl(System.Object[] args)
   at System.Delegate.DynamicInvoke(System.Object[] args)
   at GLib.Signal.ClosureInvokedCB(System.Object o,
GLib.ClosureInvokedArgs args)
   at GLib.SignalClosure.Invoke(GLib.ClosureInvokedArgs args)
   at GLib.SignalClosure.MarshalCallback(IntPtr raw_closure, IntPtr
return_val, UInt32 n_param_vals, IntPtr param_values, IntPtr
invocation_hint, IntPtr marshal_data)
   at Gtk.Application.gtk_main()
   at Gtk.Application.Run()
   at App.Main(System.String[] args)


Version-Release number of selected component (if applicable): gnutu-2.5-7


How reproducible: always


Steps to Reproduce:
1. First start
2. Next
3. Next
4. Click on "to do it click here"

Solution: update gnutu on new verion http://packages.mageialinux.ru/mageia3/SRPMS/gnutu-2.6-1.mga3.mrc.src.rpm


Reproducible: 

Steps to Reproduce: