[OpenLayers-Users] problems with a popup sans marker

[Puneet Kishor]

I am registering an click event to the map

    map.events.register("click", map, popUp);

Then querying the db for the feature at the point of click

    popUp: function(e) {
    	
    	// From google to latlng
        var lonlat = map.getLonLatFromViewPortPx(e.xy).transform(
        	proj.google,
        	proj.latlng
        );
        
        $.ajax({
	        url: "http://server/feature.json",
	        data: {lng: lonlat.lon, lat: lonlat.lat},
	        type: "GET",
	        dataType: "jsonp",
	        success: function(data) {
                var html = unravel_data(data);
		    	var c = map.getLonLatFromViewPortPx(e.xy);

		        map.addPopup(
			        new OpenLayers.Popup.Framed(
			            "featurePopup",
			            new OpenLayers.LonLat(c.lon, c.lat),
			            new OpenLayers.Size(200, 150),
			            html,
			            null,
			            false,
			            null
			        )
		        );  
			}
	    });
    }

But, instead of the popup, I get the following in my console

	OpenLayers.js: 2367: TypeError: 'null' is not an object (evaluating 'this.positionBlocks[this.relativePosition]')
	
Note that I want a popup without a marker or a vector feature. So, I have two questions --

1. How can I successfully draw the above popup?
2. Can I register the event to a layer instead of to the map? The reason is, this particular layer doesn't cover the entire map, so I want the feature query to be performed only when the user clicks on the layer.

--
Puneet Kishor