Qt Designer with QGIS 3.10.0 custom widgets will not start

[kerkovits]

The Qt designer packaged with QGIS 3.10 does not start up. However, if I delete the qgis_customwidgets.dll file, Qt designer can be opened (without the QGIS widgets). The same applies for the OSGEO4W version. The Qt designer packed with QGIS 3.8 works well. The Qt designer packed with QGIS 3.10 can not be opened even if I try to replace qgis_customwidgets.dll from an older verison of QGIS.

OS: Windows 10
QGIS: 3.10.0-3 64 bit

[jef-n]

probably introduced with 69dab87 - custom widgets are used without QgsApplication.

[jaminzoda]

Same on Ubuntu 19.10. Qt designer does not start with libqgis-customwidgets 1:3.10.0+31eoan. I tried to it with version 1:3.8.0 and it worked well.

[DavidPavlicek]

Same on Debian 10 with the latest release package. But it works again with the nightly build...

[janhec]

libqgis_customwidgets.dll was probably renamed to qgis_customwidgets.dll from at least version 3.22, I think.
Assuming that, there is no crash, but on windows 64 it will not be loaded either:
"Could not load library C:\PROGRA~1\QGIS 3.22.4\apps\qgis-ltr\qtplugins\designer\qgis_customwidgets.dll: The specified module cannot be found." (qt designer, help, about plugins).
The file is present. Elsewhere (apps\qgis-dev\qtplugins\designer) there is a slighly larger one, trying to load that one gives a slightly different error claiming a procedure (rather than a module) cannot be found.
I start qt designer from the (qgis) start-menu, so the .bat is used and environment is set.
Then I repeated this with versions 3.28.15 LTR, 3.34.5 LTR and 3.36.1.
All freshly installed. The same problem existed with all of them.
I installed qgis in WSL Ubuntu, where qt designer works without this flaw (version 3.22.4 LTR).
Is there perhaps a name mangling problem in de dll, or have different compilers (clang, mingw, VS, ???) been used for the dll vs qt designer, perhaps? I read somewhere that may cause trouble.
The QT designer version is 5.15.2 in the oldest qgis install mentioned and 5.15.3 in the others.

[jef-n]

libqgis_customwidgets.dll was probably renamed to qgis_customwidgets.dll from at least version 3.22, I think.

No, the "regular" name on Windows is qgis_customwidgets.dll (MSVC build). libqgis_customwidgets.dll is from a mingw build.

Assuming that, there is no crash, but on windows 64 it will not be loaded either: "Could not load library C:\PROGRA~1\QGIS 3.22.4\apps\qgis-ltr\qtplugins\designer\qgis_customwidgets.dll: The specified module cannot be found." (qt designer, help, about plugins).

Not reproducable with a fresh install from OSGeoW.

[janhec]

Surprising, but thanks for the quick reaction!
I used QGIS-OSGeo4W-3.34.5-1.msi and similar for the other installs.
I'll try on another machine, but no clue what happens here, since I reported exactly my results.
There is certainly no 32 vs 64 bits confusion, so I am at a loss where to look.

[janhec]

Tried on my laptop, exactly the same (negative) result. Any pointers?