Strange occurance of error: "No module named 'kivymd.button'" - python-3.x

Question

When I run my program from PyCharm, it runs without any problems. However, when building an exe using PyInstaller I am getting a very strange error, especially as this error message is referencing a line number that does not exist.
Here is the full error message:
Traceback (most recent call last):
File "ngcmain.py", line 441, in <module>
File "pdesigner.py", line 1253, in showhome
File "kivy\app.py", line 950, in run
File "kivy\base.py", line 582, in runTouchApp
File "kivy\base.py", line 347, in mainloop
File "kivy\base.py", line 391, in idle
File "kivy\base.py", line 342, in dispatch_input
File "kivy\base.py", line 308, in post_dispatch_input
File "kivy\_event.pyx", line 709, in kivy._event.EventDispatcher.dispatch
File "kivy\uix\behaviors\button.py", line 179, in on_touch_up
File "kivy\_event.pyx", line 705, in kivy._event.EventDispatcher.dispatch
File "kivy\_event.pyx", line 1248, in kivy._event.EventObservers.dispatch
File "kivy\_event.pyx", line 1132, in kivy._event.EventObservers._dispatch
File "kivy\lang\builder.py", line 57, in custom_callback
File "<string>", line 279, in <module>
File "pdesigner.py", line 201, in show_procguides
File "PyInstaller\loader\pyimod03_importers.py", line 546, in exec_module
File "procguides.py", line 18, in <module>
ModuleNotFoundError: No module named 'kivymd.button'
The error refers to line 18 in procguides.py, but this line does not exist as you can see in the following code fragment:
from kivy.uix.screenmanager import Screen
from kivy.lang import Builder
from procguideskv import style
Builder.load_string(style)
class ProcguidesScreen(Screen):
def __init__(self, **kwargs):
super(ProcguidesScreen, self).__init__(**kwargs)
def on_enter(self):
pass
Also there is no reference to kivymd.button in the procguideskv.py file:
style = r'''
<ProcguidesScreen>
name: 'ProcguidesScreen'
id: procguidesscreen
procguidesscreen:procguidesscreen
BoxLayout:
canvas.before:
Color:
rgb: 1,1,1
Rectangle:
pos: self.pos
size: self.size
'''
I am using the following packages/versions:
Python 3.9.5
Kivy 2.0.0
kivymd 0.104.2
pyinstaller 4.5.1
pyinstaller-hooks-contrib 2021.2
What am I missing here? What is going wrong?
Thanks in advance for your help. Much appreciated.