[Mac]Show a confused error when attempting to pull images from azure

[v-xuanzh]

Version: 1.29.0-insider (user setup)
Node.js: 8.9.3
Architecture: x64
Platform: MacOS
Extension:

Repro steps:

1. Sign in to Azure.
2. Choose one Azure Registry which contains pushed images.
3. Right click one image then select 'Pull image from Azure'.
4. Check the result.

Expect:
Prompt user to log in to Docker CLI if the user don't currently logged to the Docker CLI.

Actual:
Pop up the below confused error with 'Path must be a string...'