[Bugs] #1318 UNSP: Do not fail on treemodel switching

[SugarLabs Bugs]

#1318: Do not fail on treemodel switching
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    Reporter:  alsroot                    |          Owner:  tomeu            
        Type:  defect                     |         Status:  new              
    Priority:  Unspecified by Maintainer  |      Milestone:  0.86             
   Component:  sugar                      |        Version:  Git as of bugdate
    Severity:  Unspecified                |       Keywords:                   
Distribution:  Unspecified                |   Status_field:  Unconfirmed      
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 Since we have current model in treeview and new one in self._model, they
 could interfere with each other.

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Ticket URL: <http://dev.sugarlabs.org/ticket/1318>
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