NPE in 3.1.0-SNAPSHOT when no qualifier is present

[veikokaap]

When building a simple pom without a qualifier with goal released-version, then the plugin crashes with a null pointer.

Plugin declaration:

<plugin>
  <groupId>org.codehaus.mojo</groupId>
  <artifactId>build-helper-maven-plugin</artifactId>
  <version>3.1.0-SNAPSHOT</version>
  <executions>
    <execution>
      <id>released-version</id>
      <goals>
        <goal>released-version</goal>
      </goals>
    </execution>
  </executions>
</plugin>

Exception:

Caused by: java.lang.NullPointerException
    at java.util.concurrent.ConcurrentHashMap.putVal (ConcurrentHashMap.java:1011)
    at java.util.concurrent.ConcurrentHashMap.put (ConcurrentHashMap.java:1006)
    at java.util.Properties.put (Properties.java:1316)
    at org.codehaus.mojo.buildhelper.AbstractDefinePropertyMojo.defineProperty (AbstractDefinePropertyMojo.java:47)
    at org.codehaus.mojo.buildhelper.ReleasedVersionMojo.defineVersionProperty (ReleasedVersionMojo.java:83)
    at org.codehaus.mojo.buildhelper.ReleasedVersionMojo.execute (ReleasedVersionMojo.java:128)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:137)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:208)
    ...