Bug 193886 NPE is thrown when preview external CSS style in ppt[1205]
engine/org.eclipse.birt.report.engine.emitter.ppt/src/org/eclipse/birt/report/engine/emitter/ppt/PPTRender.java
String imageTitle = "slide"
				+ currentPageNum
				+ "_image"
				+ ( ++shapeCount );
		String imageName;
		if ( imageNames.containsKey( imageURI ) )
		{
			imageName = (String) imageNames.get( imageURI );
		}
		else
		{
			
			String extension = getImageExtension( imageURI );
			imageName = imageTitle + "." + extension;
			imageNames.put( imageURI, imageName );
			imageExtensions.put( imageURI, extension );
			recordFileLists( imageName );
			currentImageContents.add( imageURI );
		}
		try
		{