Bug 125420 [Commands] parameters: allow commands to specify return type
bundles/org.eclipse.core.commands/src/org/eclipse/core/commands/Command.java
"Cannot remove a null execution listener"); 
		}
		if (executionListeners != null) {
			executionListeners.remove(executionListener);
			if (executionListeners.isEmpty()) {
				executionListeners = null;
			}
		}
	}
	/**
	 * <p>
	 * Removes a state from this command. This will remove the state from the
	 * active handler, if the active handler is an instance of
	 * {@link IObjectWithState}.
	 * </p>
	 * 
	 * @param stateId
	 *            The identifier of the state to remove; must not be
	 *            <code>null</code>.
	 */
	public void removeState(final String stateId) {
		if (handler instanceof IObjectWithState) {
			((IObjectWithState) handler).removeState(stateId);
		}
		super.removeState(stateId);
	}
	/**
	 * Changes the handler for this command. This will remove all the state from
	 * the currently active handler (if any), and add it to <code>handler</code>.
	 * If debugging is turned on, then this will also print information about