Unable to find package

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I have just finished building ROS-full (Melodic Melorian) on Raspian (debian:9.4:stretch). The basic components seem to work (e.g. roscore, rosnode, rostopic), but elements that require a GUI fail.

For example rqt produces:

terminate called after throwing an instance of 'pluginlib::ClassLoaderException'
  what():  Unable to find package: qt_gui
Aborted

And rviz produces:

qt5ct: using qt5ct plugin
[ INFO] [1542173819.312609894]: rviz version 1.13.1
[ INFO] [1542173819.312952236]: compiled against Qt version 5.7.1
[ INFO] [1542173819.313012288]: compiled against OGRE version 1.9.0 (Ghadamon)
terminate called after throwing an instance of 'pluginlib::ClassLoaderException'
  what():  Unable to find package: rviz
Aborted

The environment variables look OK.

env | grep ROS
ROS_ETC_DIR=/home/pi/ros_catkin_ws/install_isolated/etc/ros
ROS_ROOT=/home/pi/ros_catkin_ws/install_isolated/share/ros
ROS_MASTER_URI=http://localhost:11311
ROS_VERSION=1
ROS_PYTHON_VERSION=2
ROS_PACKAGE_PATH=/home/pi/ros_catkin_ws/install_isolated/share
ROSLISP_PACKAGE_DIRECTORIES=
ROS_DISTRO=melodic

In the directory pointed to by ROS_PACKAGE_PATH there are directories for qt_gui and rviz, both with package.xml files which I assume is what is being sought.

Running:

catkin_find qt_gui

produces:

/home/pi/ros_catkin_ws/install_isolated/share/qt_gui

Any ideas on how to solve this problem?