fix: Line isnt drawn programmatically

[CoreySafetyIndicators]

Description

When using addNextById to connect two elements, the "end dot" appears but the line is not drawn between the two element.

Steps To Reproduce

import 'package:flutter/material.dart';
import 'package:flutter_flow_chart/flutter_flow_chart.dart';

/// Minimal reproducible example for flutter_flow_chart connection bug
class FlowChartWidget extends StatelessWidget {
  const FlowChartWidget({super.key});

  @override
  Widget build(BuildContext context) {
    final dashboard = Dashboard();

    final nodeA = FlowElement(
      position: const Offset(100, 200),
      size: const Size(160, 60),
      text: 'A',
      kind: ElementKind.rectangle,
      handlers: [
        Handler.leftCenter,
        Handler.rightCenter,
        Handler.topCenter,
        Handler.bottomCenter,
      ],
    );
    final nodeB = FlowElement(
      position: const Offset(400, 200),
      size: const Size(160, 60),
      text: 'B',
      kind: ElementKind.rectangle,
      handlers: [
        Handler.leftCenter,
        Handler.rightCenter,
        Handler.topCenter,
        Handler.bottomCenter,
      ],
    );
    dashboard.addElement(nodeA);
    dashboard.addElement(nodeB);
    // Try several connection styles
    dashboard.addNextById(
      nodeA,
      nodeB.id,
      ArrowParams(
        thickness: 8,
        color: Colors.red,
        startArrowPosition: Alignment.centerRight,
        endArrowPosition: Alignment.centerLeft,
      ),
    );
    return Scaffold(
      appBar: AppBar(title: const Text('Flow Chart Bug Demo')),
      body: Center(
        child: SizedBox(
          width: 600,
          height: 300,
          child: FlowChart(
            dashboard: dashboard,
          ),
        ),
      ),
    );
  }
}

Expected Behavior

It is expected that a line be drawn from element A to B. Dragging in the UI works without issue.

Additional Context

Flutter 3.24.3 flutter_flow_chart 3.2.3