AttributeError: 'NoneType' object has no attribute 'lookup'

I have the following code to create a dasboard. It works fine. I mean it produces the plots and figures.

wgt_schoolSelected = pn.widgets.Select(name='Okul Seçiniz:', options=list(classSchool.Name.unique()))
nQuestionsPerExam = pn.widgets.IntSlider(name='Min. Sinav Soru Sayisi', start=0, end=20, step=5, value=5)

# Visualization function that updates based on widget selections
@pn.depends(wgt_schoolSelected, nQuestionsPerExam)
def VisualizeSchoolSube(schoolVal, nQuestion):
    #number of students per each gradelevel for the selected school.
    df_school = classSchool[classSchool.Name == schoolVal]
    df = df_school.groupby('GradeLevel').size().to_frame().rename(columns={0:'Count'})
    bar_studentCountByGrade =, width=400, 
                                            line_width=0, color="#e5ae38", label="Öğrenci Sayisi")

    return bar_studentCountByGrade

wgt_gradeLevel= pn.widgets.Select(name='Kaçıncı Sınıf:', options=list(range(5, 13)))

@pn.depends(wgt_schoolSelected, nQuestionsPerExam, wgt_gradeLevel)
def VisualizeGradeLevel(schoolVal, nQuestion, gradeLevel):
    df = examTakes[examTakes.Name == schoolVal]
    df = df[df.GradeLevel == gradeLevel].groupby('ExamType')[['ExamId']].count().rename(columns={'ExamId':'count'})
    examTypeCount_plot =, width=400, 
                                       height=400, line_width=0, 
                                       color="#A1C7E0", label="Sınav Türü : ").opts(xrotation=45)

    return examTypeCount_plot

        pn.Column(pn.WidgetBox("Seçenekler", wgt_schoolSelected, nQuestionsPerExam), VisualizeSchoolSube)
        pn.Column(pn.WidgetBox("Seçenekler",  wgt_gradeLevel), VisualizeGradeLevel),

However, whenever a different item is selected from wgt_schoolSelected, it outputs an error (although the plots are updated properly). I have tried many things since yesterday but still receiving the same error. Interestingly, the same error message block is displayed four times consecutively under each other. I also could not find any related posts about this. I appreciate any help.

AttributeError                            Traceback (most recent call last)
File ~\AppData\Roaming\Python\Python39\site-packages\jupyter_bokeh\, in BokehModel._sync_model(self, _, content, _buffers)
    126 new, old, attr = content["new"], content["old"], content["attr"]
    127 submodel = self._model.select_one({"id": content["id"]})
--> 128 descriptor = submodel.lookup(content['attr'])
    129 try:
    130     descriptor._set(submodel, old, new, hint=hint, setter=self)

AttributeError: 'NoneType' object has no attribute 'lookup'

Can anyone help about this issue? I cannot find a similar issue anywhere. I am struggling a lot.

Can you share a minimal, reproducible example (MRE)?

In general, a complete script can be copied/pasted and immediately run as-is with no modifications. This is much more useful than snippets.