Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/FilterConditionHandle.java
return getStringProperty( FilterCondition.EXPR_MEMBER );
	}
	/**
	 * Sets the filter expression.
	 * 
	 * @param filterExpr
	 *            the filter expression to set
	 */
	public void setExpr( String filterExpr )
	{
		setPropertySilently( FilterCondition.EXPR_MEMBER, filterExpr );
	}
	/**
	 * Returns the operator of this filter condition. The possible values are
	 * defined in
	 * {@link org.eclipse.birt.report.model.api.elements.DesignChoiceConstants},
	 * and they are:
	 * <ul>
	 * <li><code>FILTER_OPERATOR_EQ</code>
	 * <li><code>FILTER_OPERATOR_NE</code>
	 * <li><code>FILTER_OPERATOR_LT</code>
	 * <li><code>FILTER_OPERATOR_LE</code>
	 * <li><code>FILTER_OPERATOR_GE</code>
	 * <li><code>FILTER_OPERATOR_GT</code>
	 * <li><code>FILTER_OPERATOR_BETWEEN</code>
	 * <li><code>FILTER_OPERATOR_NOT_BETWEEN</code>
	 * <li><code>FILTER_OPERATOR_NULL</code>
	 * <li><code>FILTER_OPERATOR_NOT_NULL</code>
	 * <li><code>FILTER_OPERATOR_TRUE</code>
	 * <li><code>FILTER_OPERATOR_FALSE</code>