Bug 124777 Set blank tooltip in chart title will cause exception when preview.
Description: Set blank tooltip in chart title will cause exception when preview. Steps to reproduce: 1. New a simple chart 2. Add title interactivity: mouse over/show tooltip, keep tooltip text blank and click Add button. 3. Preview the chart in viewer and move mouse over the chart title. Expected result: Nothing happen. Actual result: Unexpected exception dialog poped up.
chart/org.eclipse.birt.chart.device.svg/src/org/eclipse/birt/chart/device/svg/SVGRendererImpl.java
/**
	 * SVG Graphic context
	 */
	protected SVGGraphics2D svggc;
	
	/**
	 * Element that represents the hot spot layer
	 */
	protected Element hotspotLayer;
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.birt.chart.device.IDeviceRenderer#setProperty(java.lang.String,
	 *      java.lang.Object)
	 */
	public final void setProperty( String sProperty, Object oValue )
	{
		super.setProperty( sProperty, oValue );
		if ( sProperty.equals( IDeviceRenderer.UPDATE_NOTIFIER ) )
		{
			_iun = (IUpdateNotifier) oValue;
		}		
		else if ( sProperty.equals( IDeviceRenderer.EXPECTED_BOUNDS ) )
		{
			final Bounds bo = (Bounds) oValue;
			try
			{
				dom = createSvgDocument( bo.getWidth( ), bo.getHeight( ) );
				svggc = new SVGGraphics2D( dom );