使用D3js在线图上显示标签时出错

Question

I have a two line plot with dots drown from a tsv file with D3js. I would like to show the labels near to the dots. The tsv file is not complete and some values are missing, as follows:

year    value1    value2
....
2000    8956355 
2001    8924704 
2002    8865723 
2003    8747717 
2004    8701521 
2004    8701521 
2005        11380809
2006        10672554
2007    8513818 10394369
2008    8462607 10297716
2009    8448535 9998783
2010    8411177 9988697
2011    8024205 9491908
2012    7920080 8725402
2013    7911208 8668111
2014    7807984 8274928
2015    7747598 8027083
2016    7575879 7779103

My code for displaying the first line, the dots and the labels is as follows (the same for second line):

// line1
fw1.append("path")
    .datum(data)
        .attr("class", "line")
        .attr("fill", "none")
        .attr("stroke", "#004494")
        .attr("stroke-linejoin", "round")
        .attr("stroke-linecap", "round")
        .attr("stroke-width", 1.5)
        .attr("d", line1);
// dots for line1
fw1.selectAll(".dot")
        .data(data)
            .enter()
            .append("circle")
                .attr("class", "dot")
                .style("stroke", "#004494")
                .attr("cx", function(d) { return x1(d.year); })
                .attr("cy", function(d) { return y1(d.value1); })
                .attr("r", 3.5)
fw1.append("g")
    .classed('labels-group', true)
    .selectAll('text')
    .data(data)
    .enter()
    .append('text')
        .classed('textlbl', true)
        .attr({ 'x': function(d) { return x1(d.year);},
                    'y': function(d) { return y1(d.value1);}
        })
    .text(function(d) { return d.value1 = (d.value1==="") ? null : +d.value1; });

The two lines and the dots are correctly displayed but the code returns the following error for the labels "Uncaught TypeError: Cannot read property 'text' of null".