Fix potential nullptr de-reference

The `break` that is replaced by a `return` here would only break
out of the innermost while loop so the std::vector::end could still
become accessed after that. By returning here we completely exit
out of both nested loops and therefore don't access the
std::vector::end.

src/selectionmode/backgroundcolorhelper.cpp

@@ -56,7 +56,7 @@ void BackgroundColorHelper::slotPaletteChanged()
         while (!*i) {
             i = m_colorControlledWidgets.erase(i);
             if (i == m_colorControlledWidgets.end()) {
-                break;
+                return;
             }
         }
         setBackgroundColorForWidget(*i, m_backgroundColor);