Describe the bug
If the default Windows shell is not explicitly set in user or workspace settings, the Maven: Generate from Maven Archetype command fails with the error Command "maven.archetype.generate" fails. Cannot read property 'endsWith' of null.

To Reproduce
Steps to reproduce the behavior (using VS Code Insiders, have not tested on non-Insiders):

1. Verify that the terminal.integrated.shell.windows setting is not present in both your user and workspace settings.json files. This setting is optional and if not present, VS Code defaults to PowerShell.
2. CTRL + SHIFT + P, Maven: Generate from Maven Archetype
3. Select any archetype, I tried both maven-archetype-quickstart and maven-archetype-webapp.
4. Select a destination folder
5. See error

Expected behavior
It should not fail and detect the default. At the very least it should give an error indicating that you need to set this setting.

Environments (please complete the following information as much as possible):

• OS: Windows 10
• VS Code version: 1.36.0-insiders
• Extension version 0.17.0