InvalidUserDataException using this plugin in Gradle 5.1.1

[rmorrise]

When I apply this plugin to my project, I get the following exception:

Caused by: org.gradle.api.InvalidUserDataException: Main class name has not been configured and it could not be resolved
        at org.springframework.boot.gradle.plugin.MainClassConvention.lambda$resolveMainClass$0(MainClassConvention.java:67)
        at org.springframework.boot.gradle.plugin.MainClassConvention.resolveMainClass(MainClassConvention.java:67)
        at org.springframework.boot.gradle.plugin.MainClassConvention.call(MainClassConvention.java:62)
        at org.gradle.util.GUtil.uncheckedCall(GUtil.java:459)
        at org.gradle.api.internal.ConventionAwareHelper$2.doGetValue(ConventionAwareHelper.java:82)
        at org.gradle.api.internal.ConventionAwareHelper$MappedPropertyImpl.getValue(ConventionAwareHelper.java:126)
        at org.gradle.api.internal.ConventionAwareHelper.getConventionValue(ConventionAwareHelper.java:109)
        at org.springframework.boot.gradle.tasks.run.BootRun_Decorated.getMain(Unknown Source)
        at org.gradle.api.tasks.JavaExec.exec(JavaExec.java:77)
        at org.springframework.boot.gradle.tasks.run.BootRun.exec(BootRun.java:50)

I'll try to post a minimal test case tomorrow.

I have a multi-module project, so it could be(?) related to:
#8