How to clear a ComboBox after research in WPF?

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,