Creating topics over time fails if model has deleted topics

[aymanmh]

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Desribe the bug

If the model has some of its topics deleted (via delete_topics), calling topics_over_time fails with error:

ValueError                                Traceback (most recent call last)
Cell In[37], line 1
----> 1 topics_over_time = topic_model.topics_over_time(tweets, timestamps, nr_bins=20)

File ~/miniconda3/envs/tgram/lib/python3.13/site-packages/bertopic/_bertopic.py:900, in BERTopic.topics_over_time(self, docs, timestamps, topics, nr_bins, datetime_format, evolution_tuning, global_tuning)
    896 selection = documents.loc[documents.Timestamps == timestamp, :]
    897 documents_per_topic = selection.groupby(["Topic"], as_index=False).agg(
    898     {"Document": " ".join, "Timestamps": "count"}
    899 )
--> 900 c_tf_idf, words = self._c_tf_idf(documents_per_topic, fit=False)
    902 if global_tuning or evolution_tuning:
    903     c_tf_idf = normalize(c_tf_idf, axis=1, norm="l1", copy=False)

File ~/miniconda3/envs/tgram/lib/python3.13/site-packages/bertopic/_bertopic.py:4451, in BERTopic._c_tf_idf(self, documents_per_topic, fit, partial_fit)
   4448 if fit:
   4449     self.ctfidf_model = self.ctfidf_model.fit(X, multiplier=multiplier)
-> 4451 c_tf_idf = self.ctfidf_model.transform(X)
   4453 return c_tf_idf, words

File ~/miniconda3/envs/tgram/lib/python3.13/site-packages/sklearn/utils/_set_output.py:319, in _wrap_method_output.<locals>.wrapped(self, X, *args, **kwargs)
    317 @wraps(f)
    318 def wrapped(self, X, *args, **kwargs):
--> 319     data_to_wrap = f(self, X, *args, **kwargs)
    320     if isinstance(data_to_wrap, tuple):
    321         # only wrap the first output for cross decomposition
    322         return_tuple = (
    323             _wrap_data_with_container(method, data_to_wrap[0], X, self),
    324             *data_to_wrap[1:],
    325         )

File ~/miniconda3/envs/tgram/lib/python3.13/site-packages/bertopic/vectorizers/_ctfidf.py:113, in ClassTfidfTransformer.transform(self, X)
    110     if self.reduce_frequent_words:
    111         X.data = np.sqrt(X.data)
--> 113     X = X * self._idf_diag
    115 return X

File ~/miniconda3/envs/tgram/lib/python3.13/site-packages/scipy/sparse/_matrix.py:55, in spmatrix.__mul__(self, other)
     54 def __mul__(self, other):
---> 55     return self._matmul_dispatch(other)

File ~/miniconda3/envs/tgram/lib/python3.13/site-packages/scipy/sparse/_base.py:911, in _spbase._matmul_dispatch(self, other)
    909 if issparse(other):
    910     if N != other.shape[0]:
--> 911         raise ValueError(
    912             f"{err_prefix} (n,k={N}),(k={other.shape[0]},m)->(n,m)"
    913         )
    914     return self._matmul_sparse(other)
    916 # If it's a list or whatever, treat it like an array

ValueError: matmul: dimension mismatch with signature (n,k=30856),(k=30855,m)->(n,m)

Reproduction

from bertopic import BERTopic
import re
import pandas as pd

# Prepare data
trump = pd.read_csv('https://drive.google.com/uc?export=download&id=1xRKHaP-QwACMydlDnyFPEaFdtskJuBa6')
trump.text = trump.apply(lambda row: re.sub(r"http\S+", "", row.text).lower(), 1)
trump.text = trump.apply(lambda row: " ".join(filter(lambda x:x[0]!="@", row.text.split())), 1)
trump.text = trump.apply(lambda row: " ".join(re.sub("[^a-zA-Z]+", " ", row.text).split()), 1)
trump = trump.loc[(trump.isRetweet == "f") & (trump.text != ""), :]
timestamps = trump.date.to_list()
tweets = trump.text.to_list()

topic_model = BERTopic(verbose=True)
topics, probs = topic_model.fit_transform(tweets)

topic_model.delete_topics([7])

topics_over_time = topic_model.topics_over_time(tweets, timestamps, nr_bins=20) # throws the error
topic_model.visualize_topics_over_time(topics_over_time, top_n_topics=20)

BERTopic Version

0.17.4