[Bug] Crash when trying to paste to a cell with an empty clipboard

[itsnotmeman]

Steps to reproduce the bug

1. Clear your clipboard.
2. Paste into any PlutoGrid cell (ctrl + v)
3. The program will crash. (_TypeError (Null check operator used on a null value))

Expected results

Nothing should happen.

Actual results

The program will crash. (_TypeError (Null check operator used on a null value))

Code sample

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

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'PlutoGrid Example',
      home: Scaffold(
        appBar: AppBar(title: const Text('PlutoGrid Example')),
        body: const SimplePlutoGrid(),
      ),
    );
  }
}

class SimplePlutoGrid extends StatelessWidget {
  const SimplePlutoGrid({super.key});

  @override
  Widget build(BuildContext context) {
    // Define the columns for the grid
    final List<PlutoColumn> columns = [
      PlutoColumn(
        title: 'ID',
        field: 'id',
        minWidth: 40,
        width: 40,
        enableContextMenu: false,
        enableDropToResize: false,
        type: PlutoColumnType.number(),
      ),
      PlutoColumn(
        title: 'Name',
        field: 'name',
        type: PlutoColumnType.text(),
      ),
      PlutoColumn(
        title: 'Age',
        field: 'age',
        type: PlutoColumnType.number(),
      ),
    ];

    // Define the rows for the grid
    final List<PlutoRow> rows = [
      PlutoRow(cells: {
        'id': PlutoCell(value: 1),
        'name': PlutoCell(value: 'Alice'),
        'age': PlutoCell(value: 25),
      }),
      PlutoRow(cells: {
        'id': PlutoCell(value: 2),
        'name': PlutoCell(value: 'Bob'),
        'age': PlutoCell(value: 30),
      }),
      PlutoRow(cells: {
        'id': PlutoCell(value: 3),
        'name': PlutoCell(value: 'Charlie'),
        'age': PlutoCell(value: 35),
      }),
    ];

    return PlutoGrid(
      columns: columns,
      rows: rows,
      configuration: PlutoGridConfiguration(
        enableMoveHorizontalInEditing: true, // Allow horizontal navigation
      ),
    );
  }
}

Execution Environment

Flutter version
Flutter version is 3.24.5

PlutoGrid version
PlutoGrid version is 8.0.0

OS
Windows 11