Error with QT Library When Calling `PyPlot` Uisng `PyCall` on Windows

I'm running the demo code on the `README` file:

```
plt = pyimport("matplotlib.pyplot")
x = range(0;stop=2*pi,length=1000); y = sin.(3*x + 4*cos.(2*x));
plt.plot(x, y, color="red", linewidth=2.0, linestyle="--")
plt.show()
```

I get the following error:


Then the REPL just crashes.

Could it be a missing installation by `Conda` or something?
Using Julia 1.6 on Windows 10.

If I set:

```
mat = pyimport("matplotlib")
mat.use("TkAgg")
```

Then it works.