How to fix Python problem with embedded class definitions

Working on LeetCode #1032, I'm having a problem that appears related to having embedded classes. Here is the code:

from collections import deque

class StreamChecker:

    def __init__(self, words):
        self.RPT = StreamChecker.ReversePrefixTree( StreamChecker.ReversePrefixTreeNode )
        
    def query(self, letter):
        ...
    
    class ReversePrefixTreeNode:
        def __init__( self ):
            self.char_dict = {}      # Key is char from word, value is successor node
            self.words = []          # All words that complete at this node
        def successor( self, char ):
            return self.char_dict.get( char, None )
        def getWords( self ):  
            return self.words
        def __str__( self ):  f"[ RPTNode: char_dict = { self.char_dict }, words = { self.words } ]"

    class ReversePrefixTree:
        def __init__( self, root ):
            self.root = root
        def insert( self, word ):
            node = self.root
            for char in reversed( word ):
                if char in node.char_dict:
                    node = node.char_dict[ char ]
                else:
                    next_node = ReversePrefixTreeNode()
                    node.char_dict[ char ] = next_node
                    node = next_node
            node.words.append( word )

def tester():    
    obj = StreamChecker( [ "here", "are", "some", "words" ] )
    print( "obj", obj, "obj.RPT", obj.RPT )
    print( "obj.RPT.root", obj.RPT.root.getWords() )    <<<=========== Problem statement
    
tester()

I get the following error message:

obj <__main__.StreamChecker object at 0x00000213C5FB1710> obj.RPT <__main__.StreamChecker.ReversePrefixTree object at 0x00000213C5FB3E50>
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[22], line 41
     38     print( "obj", obj, "obj.RPT", obj.RPT )
     39     print( "obj.RPT.root", obj.RPT.root.getWords() )
---> 41 tester()

Cell In[22], line 39, in tester()
     37 obj = StreamChecker( [ "here", "are", "some", "words" ] )
     38 print( "obj", obj, "obj.RPT", obj.RPT )
---> 39 print( "obj.RPT.root", obj.RPT.root.getWords() )

TypeError: StreamChecker.ReversePrefixTreeNode.getWords() missing 1 required positional argument: 'self'

I am flummoxed by the meaning of the last sentence. Aren't I giving a value "object path" to get what I want?