-- Information about the crash:
<In detail, tell us what you were doing  when the application crashed.>

If I open the search bar (Alt+F2) and enter the search term "settings" I get a notification that baloo has crashed. After this Alt+Space or Alt+F2 stops working.
If I enter the same search term in the search bar by pressing Alt+F1 the plasma would crash with the subsequent need to restart the computer.
The problem is reproduced every time, even after reboot and is present from the moment of the installation of Kubuntu 16.04.

Application: baloo_file (5.18.0)

Qt Version: 5.5.1
Operating System: Linux 4.4.0-28-generic x86_64
Distribution: Ubuntu 16.04 LTS

-- Backtrace:
Application: Baloo File Indexing Daemon (baloo_file), signal: Segmentation fault
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[Current thread is 1 (Thread 0x7f1e167928c0 (LWP 1266))]

Thread 2 (Thread 0x7f1ccb5a2700 (LWP 1898)):
#0  0x00007f1e14894e8d in poll () at ../sysdeps/unix/syscall-template.S:84
#1  0x00007f1e11a6539c in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#2  0x00007f1e11a654ac in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#3  0x00007f1e154caa9b in QEventDispatcherGlib::processEvents (this=0x7f1cc40008c0, flags=...) at kernel/qeventdispatcher_glib.cpp:420
#4  0x00007f1e15471dea in QEventLoop::exec (this=this@entry=0x7f1ccb5a1bc0, flags=..., flags@entry=...) at kernel/qeventloop.cpp:204
#5  0x0000000000420cd8 in Baloo::FileContentIndexer::run (this=0x189b770) at ../../../src/file/filecontentindexer.cpp:71
#6  0x00007f1e15290343 in QThreadPoolThread::run (this=0x19b9c80) at thread/qthreadpool.cpp:93
#7  0x00007f1e1529384e in QThreadPrivate::start (arg=0x19b9c80) at thread/qthread_unix.cpp:331
#8  0x00007f1e13ef76fa in start_thread (arg=0x7f1ccb5a2700) at pthread_create.c:333
#9  0x00007f1e148a0b5d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109
in
Thread 1 (Thread 0x7f1e167928c0 (LWP 1266)):
[KCrash Handler]
#6  0x00007f1e12fd2cb7 in mdb_midl_xmerge () from /usr/lib/x86_64-linux-gnu/liblmdb.so.0
#7  0x00007f1e12fca76c in ?? () from /usr/lib/x86_64-linux-gnu/liblmdb.so.0
#8  0x00007f1e12fcaac9 in ?? () from /usr/lib/x86_64-linux-gnu/liblmdb.so.0
#9  0x00007f1e12fcc704 in ?? () from /usr/lib/x86_64-linux-gnu/liblmdb.so.0
#10 0x00007f1e12fd0857 in mdb_cursor_del () from /usr/lib/x86_64-linux-gnu/liblmdb.so.0
#11 0x00007f1e12fd1dc0 in ?? () from /usr/lib/x86_64-linux-gnu/liblmdb.so.0
#12 0x00007f1e1595ed8e in Baloo::DocumentDB::del (this=this@entry=0x7fff917eb520, docId=docId@entry=3414292841891843) at ../../../src/engine/documentdb.cpp:110
#13 0x00007f1e15978f8b in Baloo::WriteTransaction::removeDocument (this=0x1936790, id=id@entry=3414292841891843) at ../../../src/engine/writetransaction.cpp:128
#14 0x00007f1e15971ff9 in Baloo::Transaction::removeDocument (this=this@entry=0x7fff917eb660, id=id@entry=3414292841891843) at ../../../src/engine/transaction.cpp:235
#15 0x0000000000429632 in Baloo::MetadataMover::removeMetadata (this=this@entry=0x1889110, tr=tr@entry=0x7fff917eb660, url=...) at ../../../src/file/metadatamover.cpp:82
#16 0x000000000042a012 in Baloo::MetadataMover::moveFileMetadata (this=0x1889110, from=..., to=...) at ../../../src/file/metadatamover.cpp:53
#17 0x00007f1e154a2e4f in QtPrivate::QSlotObjectBase::call (a=0x7fff917eb7c0, r=0x7fff917ebe80, this=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qobject_impl.h:124
#18 QMetaObject::activate (sender=sender@entry=0x1951d80, signalOffset=<optimized out>, local_signal_index=local_signal_index@entry=7, argv=argv@entry=0x7fff917eb7c0) at kernel/qobject.cpp:3698
#19 0x00007f1e154a37d7 in QMetaObject::activate (sender=sender@entry=0x1951d80, m=m@entry=0x644cc0 <KInotify::staticMetaObject>, local_signal_index=local_signal_index@entry=7, argv=argv@entry=0x7fff917eb7c0) at kernel/qobject.cpp:3578
#20 0x000000000042aaba in KInotify::moved (this=this@entry=0x1951d80, _t1=..., _t2=...) at ./moc_kinotify.cpp:330
#21 0x000000000042cbff in KInotify::slotEvent (this=<optimized out>, socket=<optimized out>) at ../../../src/file/kinotify.cpp:421
#22 0x00007f1e154a2e4f in QtPrivate::QSlotObjectBase::call (a=0x7fff917eba50, r=0x1951d80, this=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qobject_impl.h:124
#23 QMetaObject::activate (sender=sender@entry=0x192f570, signalOffset=<optimized out>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0x7fff917eba50) at kernel/qobject.cpp:3698
#24 0x00007f1e154a37d7 in QMetaObject::activate (sender=sender@entry=0x192f570, m=m@entry=0x7f1e156bd780 <QSocketNotifier::staticMetaObject>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0x7fff917eba50) at kernel/qobject.cpp:3578
#25 0x00007f1e1552251e in QSocketNotifier::activated (this=this@entry=0x192f570, _t1=13) at .moc/moc_qsocketnotifier.cpp:134
#26 0x00007f1e154af47b in QSocketNotifier::event (this=0x192f570, e=<optimized out>) at kernel/qsocketnotifier.cpp:260
#27 0x00007f1e1547460c in QCoreApplication::notify (event=0x7fff917ebb10, receiver=0x192f570, this=0x7fff917ebde0) at kernel/qcoreapplication.cpp:1038
#28 QCoreApplication::notifyInternal (this=0x7fff917ebde0, receiver=0x192f570, event=event@entry=0x7fff917ebb10) at kernel/qcoreapplication.cpp:965
#29 0x00007f1e154caf45 in QCoreApplication::sendEvent (event=0x7fff917ebb10, receiver=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:224
#30 socketNotifierSourceDispatch (source=0x18870e0) at kernel/qeventdispatcher_glib.cpp:101
#31 0x00007f1e11a651a7 in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#32 0x00007f1e11a65400 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#33 0x00007f1e11a654ac in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#34 0x00007f1e154caa7f in QEventDispatcherGlib::processEvents (this=0x1887610, flags=...) at kernel/qeventdispatcher_glib.cpp:418
#35 0x00007f1e15471dea in QEventLoop::exec (this=this@entry=0x7fff917ebd20, flags=..., flags@entry=...) at kernel/qeventloop.cpp:204
#36 0x00007f1e15479e8c in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1229
#37 0x000000000041698c in main (argc=1, argv=<optimized out>) at ../../../src/file/main.cpp:88


Reproducible: Always

Steps to Reproduce:
1. Press Alt+F1 or Alt+F2 or Alt+Space
2. Enter "settings"

Actual Results:  
Baloo, krunner or plasma crash.