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
log.log( Level.SEVERE, e.getMessage( ), e );
				addException( new EngineException(
						MessageConstants.SCRIPT_EVALUATION_ERROR, expr, e ) ); 
			}
		}
		return null;
	}	
	/**
	 * evaluate conditional expression. A conditional expression can have an
	 * operator, one LHS expression, and up to two expressions on RHS, i.e.,
	 * 
	 * testExpr operator operand1 operand2 or testExpr between 1 20
	 * 
	 * Now only support comparison between the same data type
	 * 
	 * @param expr
	 *            the conditional expression to be evaluated
	 * @return a boolean value (as an Object)
	 */
	public Object evaluateCondExpr( IConditionalExpression expr )
	{
		int operator = expr.getOperator( );
		IScriptExpression testExpr = expr.getExpression( );
		IScriptExpression v1 = expr.getOperand1( );
		IScriptExpression v2 = expr.getOperand2( );
		if ( testExpr == null )
			return Boolean.FALSE;
		Object testExprValue = evaluate( testExpr.getText( ) );
		if ( IConditionalExpression.OP_NONE == operator )
		{