Blank TK Window

[Shamefaced]

Hi,
Any reason why a blank Tk() window opens up when I run my code:
Code:
for run in range(RUNS):
    waittime = Monitor2()
    checkouttime = Monitor2()
    totaltimeinshop = Monitor2()
    checkout_aisle = Simulation.Resource(AISLES)
    Simulation.initialize()
    cf = Customer_Market()
    Simulation.activate(cf, cf.run(), 0.0)
    Simulation.simulate(until=CLOSING)
    
 
    Histo = waittime.histogram(low=0.0, high=80, nbins=40)
    plt = SimPlot()
    plt.plotLine(Histo, xlab='Time (min)',ylab='Qty Customers',                    
                      title="Wait Time",
                      color="red", width=2,
                      smooth='True')  
plt.mainloop()