I received the answer of this question and the suggested solution worked with me. I think the reason because that there is a fields dictionary in the res dictionary. 

Now, as my last comment there, I needed to change the readonly property of the button 'Split' to 'True' by adding the following loop in the fields_view_get method:

nodes = doc.xpath("//button[@string='Split']")
for node in nodes: 
    node.set('readonly','1') 

I traced the code and the last line was traversed, but the button on the screen was still active.

Any help is appreciated.