This is likely happening due to the fact that you have other file types under build/classes/main
than Java class files. Currently this message is bubbling up to the console as described in this post. You can work around the issue with the following code:
tasks.withType(FindBugs) {
// file types you want to exclude e.g.
exclude '**/*.conf'
exclude '**/*.properties'
exclude '**/*.properties'
}