Ubuntu 18.04 fresh installed.

$ pkg-config --list-all 2>&1 | grep -A2 -B2 GLib

(pkg-config:31662): GLib-CRITICAL **: 06:08:21.279: g_string_insert_len: assertion 'len == 0 || val != NULL' failed

What the hell is this? Is this something we need to fix?
It happens every time I use pkg-config

I had some GLib error during installation already. But system is somehow usable. More like a GLib problem. How can we fix this? And where is the problem actually?

update according @N0rbert

#dpkg --verify

Does not yield any related file change, only some common gnome tweaking xml and conf files.
Install the libglib2.0-dev does not help either. Same error pop up after pkg-config --list-all
Reinstall with the --reinstall libglib2.0-0 libglib2.0-bin libglib2.0-data does not help either. Well the find is the same as the repo one, so I do not expect this will change anything.

1. Check system integrity

I do not see GLib-CRITICAL messages on my systems, so if unsure, then reinstall related packages with:

sudo apt-get install --reinstall libglib2.0-0 libglib2.0-bin libglib2.0-data

If problem persists, then check system integrity with:

sudo dpkg --verify

or debsums:

sudo apt-get install debsums
sudo debsums --all --changed

and reinstall packages with possible damaged/missed/changed files.

2. Install packages with *.pc files

About *.pc files. If you visit search on https://packages.ubuntu.com and evaluate search for *.pc files inside packages you will see the long list of related packages and files:

So you need to install some related dev-packages.

At least one libglib2.0-dev with glib-2.0.pc:

sudo apt-get install libglib2.0-dev

and you will get something like:

$ pkg-config --list-all | grep -A2 -B2 GLib
iso-codes iso-codes - ISO country, language, script and currency codes and translations
caja-python Caja-Python - Caja-Python Components
gobject-2.0 GObject - GLib Type, Object, Parameter and Signal Library
gmodule-export-2.0 GModule - Dynamic module loader for GLib
libpcreposix libpcreposix - PCREPosix - Posix compatible interface to libpcre
geoclue-2.0 Geoclue - The Geoinformation Service
--
adwaita-icon-theme gnome-icon-theme - A collection of icons used as the basis for GNOME themes
libpcre libpcre - PCRE - Perl compatible regular expressions C library with 8 bit character support
glib-2.0 GLib - C Utility Library
mate-keybindings mate-keybindings - Keybindings configuration for MATE applications
gmodule-2.0 GModule - Dynamic module loader for GLib
gnome-system-tools gst - Gnome System Tools
gthread-2.0 GThread - Thread support for GLib
caja-sendto caja-sendto - Extend caja-sendto through plugins
xbitmaps X bitmaps - Bitmaps that are shared between X applications
--
libpcre32 libpcre32 - PCRE - Perl compatible regular expressions C library with 32 bit character support
xkeyboard-config XKeyboardConfig - X Keyboard configuration data
gmodule-no-export-2.0 GModule - Dynamic module loader for GLib
xkbcomp xkbcomp - XKB keymap compiler

(tested on clean Ubuntu 18.04.2 LTS virtual machine)

Note: Debian and Ubuntu are not source-based GNU/Linux distributions, i.e. they are not Gentoo or Funtoo. For latter it is expected to have includes and development packages and tools preinstalled, but for Debian it is not expected.