This happens when KDevelop tries to load stale declarations, e.g. from
an unloaded language plugin.

See message:
[kdevelop(21511)/(kdevplatform.language) unknown(0): Cannot load a
top-context from file
"/home/kfunk/.cache/kdevduchain/kdevelop-{e7169c72-542e-43db-ab7d-923f7cb0d500}/topcontexts/6359"
- the required language-support for handling ID 100 is probably not
  loaded
[kdevelop(21511)/(kdevelop.plugins.clang) unknown(0): Detected an
invalid declaration for "acos"

^ That last line was added as part of this patch.

Backtrace:
0  KDevelop::Declaration::kind (this=0x0) at
/home/kfunk/devel/src/kf5/kdevplatform-stable/language/duchain/declaration.cpp:98
1  0x00007fffc2f26ef4 in (anonymous namespace)::findDeclaration
(qid=..., ctx=..., position=..., handled=Python Exception <class
'gdb.error'> There is no member named next.:
) at
/home/kfunk/devel/src/kf5/kdevelop-stable/languages/clang/codecompletion/context.cpp:513
2  0x00007fffc2f2534e in ClangCodeCompletionContext::completionItems
(this=0x7fffa4003e40, abort=@0x7fffb2def7a8: false) at
/home/kfunk/devel/src/kf5/kdevelop-stable/languages/clang/codeco
mpletion/context.cpp:1021
3  0x00007fffc2f22729 in (anonymous
namespace)::ClangCodeCompletionWorker::completionRequested
(this=0x3e72ed0, url=..., position=..., text=..., followingText=...) at
/home/kfunk/devel/src
/kf5/kdevelop-stable/languages/clang/codecompletion/model.cpp:138