I am facing same issue. I am using Fusionchart 3.3. 
Error is on line # 266 in FusionCharts.HC.js. 
Statement that is giving error is "c.paper.defs.appendChild(i)", where c.paper.defs object is null. 
  
This is the call trace: 
  
Uncaught TypeError: Cannot read property 'appendChild' of null 
window.Raphael.svg.d._setFillAndStroke @ FusionCharts.HC.js:266 
window.Raphael.svg.u.attr @ FusionCharts.HC.js:285 
V.createSlice.b @ FusionCharts.HC.Charts.js:104 
V.updateSliceConf @ FusionCharts.HC.Charts.js:90 
V.createSlice.b @ FusionCharts.HC.Charts.js:104 
kb @ FusionCharts.HC.js:239 
  
Also when I load page first time, I get "c.paper.defs" object filled as "[object SVGDefsElement]" and graph renders right. 
However when I try to rerender graph by calling "FusionCharts(chartId).render();" statement on button click, I receive "c.paper.defs" as null and hence the error.