# Buggy code
def _apply_agg_function(self, f, drop=True, *args, **kwargs):
    """Perform aggregation and combine stages based on a given function.

    Args:
        f: The function to apply to each group.

    Returns:
         A new combined DataFrame with the result of all groups.
    """
    assert callable(f), "'{0}' object is not callable".format(type(f))

    if self._is_multi_by:
        return self._default_to_pandas(f, *args, **kwargs)

    if isinstance(self._by, type(self._query_compiler)):
        by = self._by.to_pandas().squeeze()
    else:
        by = self._by

    # For aggregations, pandas behavior does this for the result.
    # For other operations it does not, so we wait until there is an aggregation to
    # actually perform this operation.
    if self._idx_name is not None and drop and self._drop:
        groupby_qc = self._query_compiler.drop(columns=[self._idx_name])
    else:
        groupby_qc = self._query_compiler
    new_manager = groupby_qc.groupby_agg(
        by, self._axis, f, self._kwargs, kwargs, drop=self._drop
    )
    if self._idx_name is not None and self._as_index:
        new_manager.index.name = self._idx_name
    result = type(self._df)(query_compiler=new_manager)
    if self._kwargs.get("squeeze", False):
        return result.squeeze()
    return result


# Traceback (Exception)
FutureWarning: pandas.core.index is deprecated and will be removed in a future version.  The public classes are available in the top-level namespace.
UserWarning: User-defined function verification is still under development in Modin. The function provided is not verified.
empty !
Traceback (most recent call last):
File "./test2.py", line 24, in <module>
rn = r.to_numpy()
File "~/.local/lib/python3.7/site-packages/modin/pandas/base.py", line 2962, in to_numpy
arr = self._query_compiler.to_numpy()
File "~/.local/lib/python3.7/site-packages/modin/backends/pandas/query_compiler.py", line 169, in to_numpy
len(arr) != len(self.index) or len(arr[0]) != len(self.columns)
File "~/.local/lib/python3.7/site-packages/modin/error_message.py", line 40, in catch_bugs_and_request_email
" caused this error.\n{}".format(extra_log)
Exception: Internal Error. Please email bug_reports@modin.org with the traceback and command that caused this error.

# Fixed code
def _apply_agg_function(self, f, drop=True, *args, **kwargs):
    """Perform aggregation and combine stages based on a given function.

    Args:
        f: The function to apply to each group.

    Returns:
         A new combined DataFrame with the result of all groups.
    """
    assert callable(f), "'{0}' object is not callable".format(type(f))

    f = wrap_udf_function(f)
    if self._is_multi_by:
        return self._default_to_pandas(f, *args, **kwargs)

    if isinstance(self._by, type(self._query_compiler)):
        by = self._by.to_pandas().squeeze()
    else:
        by = self._by

    # For aggregations, pandas behavior does this for the result.
    # For other operations it does not, so we wait until there is an aggregation to
    # actually perform this operation.
    if self._idx_name is not None and drop and self._drop:
        groupby_qc = self._query_compiler.drop(columns=[self._idx_name])
    else:
        groupby_qc = self._query_compiler
    new_manager = groupby_qc.groupby_agg(
        by, self._axis, f, self._kwargs, kwargs, drop=self._drop
    )
    if self._idx_name is not None and self._as_index:
        new_manager.index.name = self._idx_name
    result = type(self._df)(query_compiler=new_manager)
    if self._kwargs.get("squeeze", False):
        return result.squeeze()
    return result