Whenever I start baloo, I am getting the following error ...

kubuntu@kubuntu-ThinkPad-L460:~$ balooctl start
kubuntu@kubuntu-ThinkPad-L460:~$ org.kde.baloo: "/home/kubuntu"
org.kde.baloo: "/media/kubuntu/PLAY"
org.kde.baloo: "/media/kubuntu/WORK"
Power state changed
mdb.c:2065: Assertion 'rc == 0' failed in mdb_page_dirty()
KCrash: Attempting to start /usr/bin/baloo_file from kdeinit
sock_file=/run/user/1000/kdeinit5__0
KCrash: crashing... crashRecursionCounter = 2
KCrash: Application Name = baloo_file path = /usr/bin pid = 372
KCrash: Arguments: /usr/bin/baloo_file
KCrash: Attempting to start /usr/lib/x86_64-linux-gnu/libexec/drkonqi from kdeinit
sock_file=/run/user/1000/kdeinit5__0
QSocketNotifier: Invalid socket 4 and type 'Read', disabling...
QSocketNotifier: Invalid socket 8 and type 'Exception', disabling...
QSocketNotifier: Invalid socket 7 and type 'Read', disabling...
QSocketNotifier: Invalid socket 14 and type 'Read', disabling...

As a result, I am no longer able to get any search results in krunner. Please advice.

Bump! Anybody have a clue what to do here?

It seems you are not alone:
https://bugs.kde.org/show_bug.cgi?id=362222

But to be actually sure your issue is the same we would need a backtrace from DrKonqi.

I get the exact same problem here:

zubin@zubin-HP-Z420-Workstation:~$ balooctl start
zubin@zubin-HP-Z420-Workstation:~$ Power state changed
mdb.c:2065: Assertion 'rc == 0' failed in mdb_page_dirty()
KCrash: Attempting to start /usr/bin/baloo_file from kdeinit
sock_file=/run/user/1000/kdeinit5__0
KCrash: crashing... crashRecursionCounter = 2
KCrash: Application Name = baloo_file path = /usr/bin pid = 20035
KCrash: Arguments: /usr/bin/baloo_file
KCrash: Attempting to start /usr/lib/x86_64-linux-gnu/libexec/drkonqi from kdeinit
sock_file=/run/user/1000/kdeinit5__0
QSocketNotifier: Invalid socket 6 and type 'Read', disabling...
QSocketNotifier: Invalid socket 11 and type 'Read', disabling...
QSocketNotifier: Invalid socket 10 and type 'Read', disabling...
QSocketNotifier: Invalid socket 13 and type 'Exception', disabling...
QSocketNotifier: Invalid socket 12 and type 'Read', disabling...
QSocketNotifier: Invalid socket 15 and type 'Read', disabling...

zubin@zubin-HP-Z420-Workstation:~$ dpkg -l | grep -i baloo
ii baloo-kf5 5.45.0-0neon+16.04+xenial+build55 amd64 framework for searching and managing metadata
ii libkf5baloo5 5.45.0-0neon+16.04+xenial+build55 amd64 framework for searching and managing metadata core lib.
ii libkf5balooengine5 5.45.0-0neon+16.04+xenial+build55 amd64 framework for searching and managing metadata plugins
ii libkf5baloowidgets-bin 4:18.04.0-0neon+16.04+xenial+build36 amd64 Wigets for use with Baloo - binaries
ii libkf5baloowidgets5:amd64 4:18.04.0-0neon+16.04+xenial+build36 amd64 Wigets for use with Baloo