How to clear a ComboBox after research in WPF?

Question

I had a research by criteria with Two combobox, it works fine after the research is finished, I have a button Display All : to reset the combobox to null..and the DataGrid display with all elements ,

The problem that the combobox must be empty when I click on the Button Dispaly All, but the twice try is correct, after that...I have the list of two combobox are Empty!!

• Without select an element in combobox(just dispaly the datagrid):I have 6 elements in the datagrid, it is correct..and the combobox are Empty :
  
  
  
  
• After select the Search criteria, i have the result correct: (I have just 3 results, it is the correct action) :
  
  
  
  
• When I click on the button Display All (after twice try ):the list in the Combobox are Empty!! :
  
  
  

How can I fix it ?

Thanks,