Maven clean command: java.util.Collections.UnmodifiableRandomAccessList to property of type java.util.ArrayList

I had the same issue with Maven 3.6.0 as well. The problem seems to be a bug in the plugin, because it disappears starting from the plugin version 3.0.4. So the solution for me was to specify the version in the pom.xml:

  <build>
    <pluginManagement>
      <plugins>
        <plugin>
          <groupId>org.codehaus.mojo</groupId>
          <artifactId>findbugs-maven-plugin</artifactId>
          <version>3.0.4</version>
          <configuration>
            <failOnError>${maven.findbugs.failure.strict}</failOnError>
          </configuration>
        </plugin>
      </plugins>
    </pluginManagement>
  </build>

Using mvn version 3.3.x might solve this problem. I got this error with 3.6.x.