#0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44                                                                
44	      return INTERNAL_SYSCALL_ERROR_P (ret) ? INTERNAL_SYSCALL_ERRNO (ret) : 0;
[Current thread is 1 (Thread 0x7fbe98ff96c0 (LWP 290551))]
(gdb) bt
#0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44
#1  0x00007fbf04a7ed03 in __pthread_kill_internal (threadid=<optimized out>, signo=6) at pthread_kill.c:78
#2  0x00007fbf04a25d1e in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#3  0x00007fbf04a0d942 in __GI_abort () at abort.c:79
#4  0x00007fbf04c3ebb0 in std::__glibcxx_assert_fail (file=file@entry=0x7fbf07e74b08 "/usr/include/c++/14/bits/stl_vector.h", line=line@entry=1213, 
    function=function@entry=0x7fbf07e79690 "std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::front() [with _Tp = unsigned int; _Alloc = std::allocator<unsigned int>; reference = unsigned int&]", condition=condition@entry=0x7fbf07e6f0ff "!this->empty()") at ../../../../../libstdc++-v3/src/c++11/assert_fail.cc:41
#5  0x00007fbf07d7234e in std::vector<unsigned int, std::allocator<unsigned int> >::front (this=<optimized out>) at /usr/include/c++/14/bits/stl_vector.h:1211
#6  std::vector<unsigned int, std::allocator<unsigned int> >::front (this=0x7fbe98ff4810) at /usr/include/c++/14/bits/stl_vector.h:1211
#7  simgear::effect::computeMipmap (image=0x7fbe0f628740, attrs=std::tuple containing = {...}) at /usr/src/debug/SimGear-2020.3.19-4.fc41.x86_64/simgear/scene/material/mipmap.cxx:306
#8  0x00007fbf07d82c7d in simgear::ModelRegistry::readImage (this=<optimized out>, fileName=..., opt=<optimized out>)
    at /usr/src/debug/SimGear-2020.3.19-4.fc41.x86_64/simgear/scene/model/ModelRegistry.cxx:595
#9  0x00007fbf07fc4607 in osgDB::Registry::readImage (this=<optimized out>, fileName="/usr/share/flightgear/Aircraft/Generic/Effects/null_bumpspec.png", options=0x7fbe400008e0)
    at /usr/src/debug/OpenSceneGraph-3.6.5-29.fc41.x86_64/include/osgDB/Registry:221
#10 osgDB::readRefImageFile (filename="/usr/share/flightgear/Aircraft/Generic/Effects/null_bumpspec.png", options=options@entry=0x7fbe400008e0)
    at /usr/src/debug/OpenSceneGraph-3.6.5-29.fc41.x86_64/src/osgDB/ReadFile.cpp:106
#11 0x00007fbf07d59a67 in simgear::(anonymous namespace)::setAttrs (attrs=std::tuple containing = {...}, tex=0x7fbe0f4b9130, options=0x7fbe400008e0)
    at /usr/src/debug/SimGear-2020.3.19-4.fc41.x86_64/simgear/scene/material/TextureBuilder.cxx:282
#12 0x00007fbf07d619db in simgear::TexBuilder<osg::Texture2D>::build (this=0x56046f17ed70, effect=<optimized out>, pass=<optimized out>, props=<optimized out>, options=0x7fbe400008e0)
    at /usr/src/debug/SimGear-2020.3.19-4.fc41.x86_64/simgear/scene/material/TextureBuilder.cxx:354
#13 0x00007fbf07d59758 in simgear::TextureUnitBuilder::buildAttribute (this=<optimized out>, effect=0x7fbe0f5b5f60, pass=0x7fbe0f625950, prop=0x7fbe0f5e06d0, options=0x7fbe400008e0)
    at /usr/src/debug/SimGear-2020.3.19-4.fc41.x86_64/simgear/scene/material/TextureBuilder.cxx:139

This is because the bumpspec png is 1x1 and the code expects 2x2 as a minimum, but does not raise any error.
Enlarging the texture to 2x2 (or any power of 2) fixes the crash.

This can be done in two ways:

1/ With ImageMagic as a build dep:
$ magick null_bumpspec.png -resize 2 null_bumpspec.png

2/ by including upstream's new null_bumpspec.png in the lookaside cache, as the new file is 32x32:
https://sourceforge.net/p/flightgear/fgdata/ci/next/tree/Aircraft/Generic/Effects/null_bumpspec.png?format=raw

I am partial to doing 1 as it solves the crash without adding binary data that won't be reused. 

I can co-maint if you like. 

Reproducible: Always

Steps to Reproduce:
1. Launch fgfs twice, watch it crash the second time

Note that using --ignore-autosave makes the problem go away.