Fix error after flutter pub downgrade.

[timbotimbo2]

Description

#1034 works fine after a flutter pub get but introduced an error if you run flutter pub downgrade.

ARGUMENT_TYPE_NOT_ASSIGNABLE - lib/src/web/web_unity_widget_controller.dart:259:44 - The argument type 'Element' can't be assigned to the parameter type 'JSAny?'`

This is a quick fix.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore