diff --git a/src/kmainwindow.cpp b/src/kmainwindow.cpp
--- a/src/kmainwindow.cpp
+++ b/src/kmainwindow.cpp
@@ -239,6 +239,23 @@
 
     sMemberList()->append(q);
 
+    // Set the icon theme fallback to breeze (if not already set)
+    // Most of our apps use "lots" of icons that most of the times
+    // are only available with breeze, we still honour the user icon
+    // theme but if the icon is not found there, we go to breeze
+    // since it's almost sure it'll be there.
+    // This should be done as soon as possible (preferably via
+    // Q_COREAPP_STARTUP_FUNCTION), but as for now it cannot be done too soon
+    // as at that point QPlatformTheme is not instantiated yet and breaks the
+    // internal status of QIconLoader (see QTBUG-74252).
+    // See also discussion at https://phabricator.kde.org/D22488
+    // TODO: remove this once we depend on Qt 5.15.1, where this is fixed
+#if QT_VERSION >= QT_VERSION_CHECK(5, 12, 0)
+    if (QIcon::fallbackThemeName().isEmpty()) {
+        QIcon::setFallbackThemeName(QStringLiteral("breeze"));
+    }
+#endif
+
     // If application is translated, load translator information for use in
     // KAboutApplicationDialog or other getters. The context and messages below
     // both must be exactly as listed, and are forced to be loaded from the