[Question] Error building Flet app on Windows: Issues with return types in theme.dart

[Duz-Dev]

Question

Hi, I’m trying to build a Flet app on Windows using the flet build windows command, but I’m encountering errors related to return types in the theme.dart file. It seems there’s an incompatibility between Flutter dependencies and the expected API types.

---

Details:
When I run the command flet build windows, I receive the following error log:

[17:04:41] Created Flutter bootstrap project from gh:flet-dev/flet-build-template with ref 0.25.2 ✅
           Customized app icons and splash images ✅
[17:04:48] Generated app icons ✅
[17:05:10] Packaged Python app ✅
[17:05:51] Resolving dependencies...
           ...
           ../../../../AppData/Local/Pub/Cache/hosted/pub.dev/flet-0.25.2/lib/src/utils/theme.dart(356,28): error
           G44692867: A value of type 'TabBarThemeData' can't be returned from a function with return type
           'TabBarTheme?'.
           [C:\Users\miner\DuzDev\prueba\build\flutter\build\windows\x64\flutter\flutter_assemble.vcxproj]
           ../../../../AppData/Local/Pub/Cache/hosted/pub.dev/flet-0.25.2/lib/src/utils/theme.dart(438,28): error
           G44692867: A value of type 'DialogThemeData' can't be returned from a function with return type
           'DialogTheme?'.
           [C:\Users\miner\DuzDev\prueba\build\flutter\build\windows\x64\flutter\flutter_assemble.vcxproj]
           ../../../../AppData/Local/Pub/Cache/hosted/pub.dev/flet-0.25.2/lib/src/utils/theme.dart(482,26): error
           G44692867: A value of type 'CardThemeData' can't be returned from a function with return type 'CardTheme?'.
           [C:\Users\miner\DuzDev\prueba\build\flutter\build\windows\x64\flutter\flutter_assemble.vcxproj]
           ...
           Build process failed.

---

My environment:

• Operating System: Windows 11
• Flutter Version: 3.27.0 (Stable)
• Flet Version: 0.25.2
• Visual Studio Version: Community 2022 17.12.3
• Python Version: 3.12.3 with venv environment enabled

---

Steps I’ve tried to resolve the issue:

1. Ran flutter pub upgrade to update dependencies.
2. Checked for Flet updates using pip install --upgrade flet.
3. Attempted manual fixes in the theme.dart file by modifying return types, but I’d prefer an official solution.

---

Request for help:
Is there a fix or update planned for this issue? Could it be related to Flutter 3.27.0 compatibility? If someone has encountered a similar issue, I’d appreciate any guidance or a workaround.

Thank you for your support!

Code sample

No response

Error message

(.venv) PS C:\Users\miner\DuzDev\prueba> flet build windows
[17:04:41] Created Flutter bootstrap project from gh:flet-dev/flet-build-template with ref 0.25.2 ✅
           Customized app icons and splash images ✅
[17:04:48] Generated app icons ✅
[17:05:10] Packaged Python app ✅
[17:05:51] Resolving dependencies...
           Downloading packages...
             archive 3.6.1 (4.0.1 available)
             characters 1.3.0 (1.4.0 available)
             clock 1.1.1 (1.1.2 available)
             collection 1.19.0 (1.19.1 available)
           ! file_picker 8.1.4 (overridden) (8.1.5 available)
             flutter_lints 2.0.3 (5.0.0 available)
             image 4.3.0 (4.5.0 available)
             intl 0.19.0 (0.20.1 available)
             js 0.6.7 (0.7.1 available)
             lints 2.1.1 (5.1.0 available)
             material_color_utilities 0.11.1 (0.12.0 available)
             meta 1.15.0 (1.16.0 available)
             path 1.9.0 (1.9.1 available)
             sensors_plus 4.0.2 (6.1.1 available)
             sensors_plus_platform_interface 1.2.0 (2.0.1 available)
             serious_python 0.8.3 (0.8.4 available)
             toml 0.15.0 (0.16.0 available)
             url_strategy 0.2.0 (discontinued)
           ! wakelock_plus 1.2.8 (overridden)
           ! web 1.1.0 (overridden)
             web_socket_channel 2.4.5 (3.0.1 available)
           ! window_manager 0.4.3 (overridden)
           Got dependencies!
           1 package is discontinued.
           19 packages have newer versions incompatible with dependency constraints.
           Try `flutter pub outdated` for more information.

           windows/flutter/CMakeLists.txt does not use FLUTTER_TARGET_PLATFORM, updating.
           Building Windows application...
           Building Windows application...                                    37.8s

           ../../../../AppData/Local/Pub/Cache/hosted/pub.dev/flet-0.25.2/lib/src/utils/theme.dart(356,28): error
           G44692867: A value of type 'TabBarThemeData' can't be returned from a function with return type
           'TabBarTheme?'.
           [C:\Users\miner\DuzDev\prueba\build\flutter\build\windows\x64\flutter\flutter_assemble.vcxproj]
           ../../../../AppData/Local/Pub/Cache/hosted/pub.dev/flet-0.25.2/lib/src/utils/theme.dart(438,28): error
           G44692867: A value of type 'DialogThemeData' can't be returned from a function with return type
           'DialogTheme?'.
           [C:\Users\miner\DuzDev\prueba\build\flutter\build\windows\x64\flutter\flutter_assemble.vcxproj]
           ../../../../AppData/Local/Pub/Cache/hosted/pub.dev/flet-0.25.2/lib/src/utils/theme.dart(482,26): error
           G44692867: A value of type 'CardThemeData' can't be returned from a function with return type 'CardTheme?'.
           [C:\Users\miner\DuzDev\prueba\build\flutter\build\windows\x64\flutter\flutter_assemble.vcxproj]
           C:\Program Files\Microsoft Visual
           Studio\2022\Community\MSBuild\Microsoft\VC\v170\Microsoft.CppCommon.targets(254,5): error MSB8066:
           Compilación personalizada de
           "C:\Users\miner\DuzDev\prueba\build\flutter\build\windows\x64\CMakeFiles\32618c91ec424f4b21051be03aaacc60\f
           lutter_windows.dll.rule;C:\Users\miner\DuzDev\prueba\build\flutter\build\windows\x64\CMakeFiles\97a3af39dde
           e685113b99ae8d51321ce\flutter_assemble.rule;C:\Users\miner\DuzDev\prueba\build\flutter\windows\flutter\CMak
           eLists.txt" terminada con el código 1.
           [C:\Users\miner\DuzDev\prueba\build\flutter\build\windows\x64\flutter\flutter_assemble.vcxproj]
           Build process failed.

           Error building Flet app - see the log of failed command above.
[17:05:56] Doctor summary (to see all details, run flutter doctor -v):
           [✓] Flutter (Channel stable, 3.27.0, on Microsoft Windows [Versi¢n 10.0.22631.4460], locale es-MX)
           [✓] Windows Version (Installed version of Windows is version 10 or higher)
           [✓] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
           [✓] Chrome - develop for the web
           [✓] Visual Studio - develop Windows apps (Visual Studio Community 2022 17.12.3)
           [✓] Android Studio (version 2024.2)
           [✓] VS Code (version 1.95.2)
           [✓] Connected device (3 available)
           [✓] Network resources

           • No issues found!

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• I have searched for answers to my question both in the issues and in previous discussions.

[ali19991377]

I have the same problem. just downgrade your flutter to version 3.24.0. it would be solved.

[ndonkoHenri]

#4548