Hello, I have the following task:

  <target name="composer:validate" description="Check composer.json syntax">
    <composer>
      <arg value="validate"/>
      <arg value="--strict"/>
      <arg value="--no-check-lock"/>
    </composer>
  </target>

This task ran perfectly in Phing 3.0.0-alpha2, but in Phing 3.0.0-alpha3 I get the following error message:

Argument 1 passed to CommandlineArgument::setValue() must be of the type string, null given, called in phar:///usr/bin/phing/classes/phing/tasks/ext/ComposerTask.php on line 170

To solve this issue I have to refactor the target as this:

  <target name="composer:validate" description="Check composer.json syntax">
    <composer command="validate">
      <arg value="--strict"/>
      <arg value="--no-check-lock"/>
    </composer>
  </target>

I agree with command property being required, but if so then ComposerTask's documentation and schema should be updated too. Also error message should be gracefully handled instead of printing the exception.

Let me know if you want me to do it :)