Fix an issue while calling onTap: close #8

[kcoms555]

Summary

• onTap function #8 onTap is not working as described

Problems

flutter-interactive-chart/lib/src/interactive_chart.dart

Lines 214 to 218 in 8bce5d6

	onTapUp: (_) {
	setState(() => _tapPosition = null);
	// Fire callback event (if needed)
	if (widget.onTap != null) _fireOnTapEvent();
	},

• _fireOnTapEvent() needs position information to get a position of a candle.
• But assignment of null on _tapPosition inside setState() make it unavailable.

Expected Results

• make onTap() work.