Maven: "Theme name and theme class can not both be specified" with Material theme and themeFolder

[mvysny]

Description of the bug

Vaadin fails to build the production bundle for the annotation @Theme(value = Material.class, themeFolder = "my-theme"), with a strange error message:

[ERROR] Failed to execute goal com.vaadin:vaadin-maven-plugin:14.9.5:build-frontend (default) on project project-base: Execution default of goal com.vaadin:vaadin-maven-plugin:14.9.5:build-frontend failed: Theme name and theme class can not both be specified. Theme name uses Lumo and can not be used in combination with custom theme class that doesn't extend Lumo. -> [Help 1]

If the themeFolder attribute isn't supported in connection with the Material theme, the error message should mention themeFolder instead of something called 'theme name'; or even better, the error message should mention that "Only the Lumo theme supports custom themes".

Expected behavior

Vaadin should either support Material custom theme according to the https://vaadin.com/docs/v14/flow/styling/custom-theme manual, or Maven should fail with a clearer error message.

Minimal reproducible example

1. clone skeleton-starter-flow for Vaadin 14
2. add @Theme(value = Material.class, themeFolder = "my-theme") to the MainView class
3. mvn -C clean package -Pproduction

Versions

• Vaadin / Flow version: 14.9.5
• Java version: 11

[knoobie]

Looking at the linked docs they state as limitation:

Using the built-in Material theme is not currently supported. Custom themes are always loaded on top of the Lumo theme.

[mvysny]

Yup, you're right, the documentation clears up everything. Still, the Maven plugin error message could be improved to mention the custom themes and Lumo. Essentially, the same line as the one from the documentation would be great.