Fix: null labelPainter

lib/src/animated_circular_chart.dart

@@ -243,7 +243,7 @@ class AnimatedCircularChartState extends State<AnimatedCircularChart>
       size: widget.size,
       painter: new AnimatedCircularChartPainter(
         _tween.animate(_animation),
-        _labelPainter,
+        widget.holeLabel != null ? _labelPainter : null,
       ),
     );
   }

lib/src/painter.dart

@@ -14,12 +14,7 @@ class AnimatedCircularChartPainter extends CustomPainter {
 
   @override
   void paint(Canvas canvas, Size size) {
-    labelPainter.paint(
-        canvas,
-        new Offset(
-          size.width / 2 - labelPainter.width / 2,
-          size.height / 2 - labelPainter.height / 2,
-        ));
+    _paintLabel(canvas, size, labelPainter);
     _paintChart(canvas, size, animation.value);
   }
 
@@ -35,12 +30,7 @@ class CircularChartPainter extends CustomPainter {
 
   @override
   void paint(Canvas canvas, Size size) {
-    labelPainter.paint(
-        canvas,
-        new Offset(
-          size.width / 2 - labelPainter.width / 2,
-          size.height / 2 - labelPainter.height / 2,
-        ));
+    _paintLabel(canvas, size, labelPainter);
     _paintChart(canvas, size, chart);
   }
 
@@ -50,6 +40,18 @@ class CircularChartPainter extends CustomPainter {
 
 const double _kRadiansPerDegree = Math.pi / 180;
 
+void _paintLabel(Canvas canvas, Size size, TextPainter labelPainter) {
+  if (labelPainter != null) {
+    labelPainter.paint(
+      canvas,
+      new Offset(
+        size.width / 2 - labelPainter.width / 2,
+        size.height / 2 - labelPainter.height / 2,
+      ),
+    );
+  }
+}
+
 void _paintChart(Canvas canvas, Size size, CircularChart chart) {
   final Paint segmentPaint = new Paint()
     ..style = chart.chartType == CircularChartType.Radial