Callback triggers but returns nothing

So I’m getting several similar callback errors but the functionality on the webapp still works as intended. Im still trying to remove those error messages though:

Error message: TypeError: only list-like objects are allowed to be passed to isin(), you passed a [NoneType]

So this is simple. In the following callback the fourth input is None when the app refreshes, or another dropdown is blank.
When I populate that other filter, i dont get this error message.

# populate time filter dropdown
@app.callback(
    Output('time-filter', 'options'),
    [Input('datasets', 'data'),
     Input('sim-filter', 'value'),
     Input('source-filter', 'value'),
     Input('event-filter', 'value')]
)
def update_time_filter(datasets_store, selected_sim, selected_source, selected_event):
    # FIXME: NoneType error in if statement
    datasets = json.loads(datasets_store)
    df_ec = pd.read_json(datasets['df_ec'], orient='split')
    df_ec = df_ec[(df_ec.SimId == selected_sim)
                  & (df_ec.Source == selected_source)
                  & (df_ec.EventType.isin(selected_event))]
    times = [{'label': str(simtime), 'value': simtime} for simtime in sorted(df_ec.SimulationTime.unique())]
    times.insert(0, {'label': 'Select all', 'value': 'all_values'})  
    return times

So I tried several things:
If statement with dash.no_update, dash.exceptions. PreventUpdate, pass, return None and by doing any of these things, my dropdown callback doesnt work anymore.

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