Bug 165278 this.getDisplayContent()="" in script of text will destroy preview[0801]
Description: this.getDisplayContent()="" in script of text will destroy preview Build number:2.1.2.v20061120 and 2.2.0.v20061117 Steps to reproduce: 1. Insert a text item, select it and switch to script editor 2. Input this.getDisplayContent()="" and preview Expected result: Error message about the incorrect script. Actual result: In 2.1.2, I saw HTTP Status 500 - type Exception report message description The server encountered an internal error () that prevented it from fulfilling this request. In 2.2.0, I saw error dialog with java.lang.ClassCastException.
engine/org.eclipse.birt.report.engine/src/org/eclipse/birt/report/engine/executor/ExecutionContext.java
* Report parameters used to create the report content
	 */
	private Map params = new BirtHashMap( );
	private Map persistentBeans = new HashMap( );
	private Map transientBeans = new HashMap( );
	private ReportDocumentWriter docWriter;
	/**
	 * app context
	 */
	private Map appContext;
	/**
	 * report context used to evaluate the java-based script.
	 */
	private IReportContext reportContext;
	/**
	 * options used to render the report content
	 */
	private IRenderOption renderOption;
	/**
	 * the current locale, used to localize the report content
	 */
	private Locale locale;
	
	/**
	 * report document, may be the output or input.
	 */
	private IReportDocument reportDoc;
	/**
	 * the created report content
	 */