I just checked the project and it works on my end. Are you sure the service is running:

jupyter kernelgateway --KernelGatewayApp.api='kernel_gateway.notebook_http' --KernelGatewayApp.ip=0.0.0.0 --KernelGatewayApp.port=9090 --KernelGatewayApp.seed_uri=flower_power.ipynb --KernelGatewayApp.allow_origin='*'

It looks like it might be from your image, but hard to tell. Also see some warnings there with respect to TF version?

Can you run inspect element in your browser and see if there are any errors.