[Tutor] Pygubu designer question

[Phil]

I've spent most of the afternoon experimenting with Pygubu builder. I 
haven't done much with Tkinter so my knowledge is a bit thin. Pygubu 
uses a callback mechanism rather than the bind method that I'm more 
familiar with. What I'm trying to achieve is read the contents of an 
edit box when returned is pressed. I've done this in the past with 
"entry.bind('Return', on_change)" if I remember correctly.

This is taken from the file that Pygubu generates:

        self.entry1.configure(exportselection='true', relief='flat', 
takefocus=False, validate='focusout')

         self.entry1.delete('0', 'end')
         self.entry1.insert('0', _text_)
         self.entry1.place(anchor='sw', height='30', width='100', 
x='10', y='100')
         self.entry1.configure(validatecommand=self.callback)

There are other entry1 options but I think validatecommand is all that I 
need. I've searched the Internet for any clues but haven't turned up 
anything relevant.

My programme includes the following:

# define the function callbacks
def on_button1_click():
     messagebox.showinfo('Message', 'You pressed me') # no problems here

def callback(self):
     #print(entry1.get()) # entry1 is unknown and self.entry1 didn't 
help but that's another problem

     print('here')


         # Configure callbacks
         callbacks = {
             'on_button1_clicked': on_button1_click,
             'callback' : callback # This doesn't look to be correct
         }

         builder.connect_callbacks(callbacks)

The word 'here' is printed when the programme is first run. I press the 
button so that the entry box loses focus and that's when the error 
message is displayed.

The error message is Exception in Tkinter callback
Traceback (most recent call last):
   File "/usr/lib/python3.9/tkinter/__init__.py", line 1892, in __call__
     return self.func(*args)
TypeError: 'NoneType' object is not callable

I know that I need to read up on Tkinter but someone who may be familiar 
with callbacks and Pygubu may be able to help.

-- 
Regards,
Phil