I am getting this errorRuntime Error: KeyNotFoundException : ' ' wasn't found in the Slice object, likely because there was no-data at this moment in time and it wasn't possible to fillforward historical data. Please check the data exists before accessing it with data.ContainsKey(" ")
Here is where it occursself.AddEquity("TSLA", Resolution.Minute) # This is in the def initialize() function
    def OnData(self, slice):
        bars = slice.Bars
        bar = (float)(slice["TSLA"].Open)

Sometimes this error occurs, and sometimes it does not. I am not sure why it has been inconsistent. Is anybody familiar with what causes this error?