[Solved] Tray is missing

Tray icons do not appear, making harder to use applications having tray icons.

Example 1: Telegram Desktop (Flathub package "org.telegram.desktop")

$ flatpak run org.telegram.desktop
QPainter::begin: Paint device returned engine == 0, type: 2
QWidget::render: Cannot render with an inactive painter
Cannot find EGLConfig, returning null config
qt.qpa.wayland: Non-blocking swap buffers not supported. Subsurface rendering can be affected. It may also cause the event loop to freeze in some situations
Cannot find EGLConfig, returning null config
qt.qpa.wayland: Non-blocking swap buffers not supported. Subsurface rendering can be affected. It may also cause the event loop to freeze in some situations
Cannot find EGLConfig, returning null config
qt.qpa.wayland: Could not create EGL surface (EGL error 0x3005)

Cannot find EGLConfig, returning null config
qt.qpa.wayland: Non-blocking swap buffers not supported. Subsurface rendering can be affected. It may also cause the event loop to freeze in some situations
QOpenGLWidget: Failed to make context current
QRhiGles2: Context is lost.
QDBusTrayIcon encountered a D-Bus error: QDBusError("org.freedesktop.DBus.Error.ServiceUnknown", "The name org.kde.StatusNotifierWatcher was not provided by any .service files")

Example 2: KDE Connect (APT package "kdeconnect")

After this, nothing appears:

$ kdeconnect-indicator

Example 3: Clementine (APT package "clementine")

Tools -> Preferences -> Behavior: option "Show tray icon" is unticked and grayed out

How can I use tray icons?