The IntelliJ Platform Plugin SDK provides an extension point (com.intellij.diagram.Provider) to extend the IDE's UML class diagram functionality.
However when attempting to build a plugin using this extension point, Gradle cannot find the com.intellij.diagram class.
java.lang.IllegalArgumentException: Could not find bundled plugin with ID: 'com.intellij.diagram'. See https://jb.gg/ij-plugin-dependencies.
The UML class diagram functionality is only available in the Ultimate edition of the JetBrains IDEs.
Using the gradle.properties file provided in the quickstart template, it is possible to change which edition the plugin is built against, which allows you to import the platform bundled plugin com.intellij.diagram.
platformType = IU # IntelliJ IDEA Ultimate
platformBundledPlugins = com.intellij.diagram