fix invisible text in field expression widget in dark themes (#9733)

backport of #9701

src/gui/qgsfieldexpressionwidget.cpp

@@ -70,6 +70,8 @@ QgsFieldExpressionWidget::QgsFieldExpressionWidget( QWidget *parent )
                      << QgsExpressionContextUtils::projectScope( QgsProject::instance() );
 
   mCombo->installEventFilter( this );
+
+  mComboPalette = mCombo->lineEdit()->palette();
 }
 
 void QgsFieldExpressionWidget::setExpressionDialogTitle( const QString &title )
@@ -336,7 +338,7 @@ void QgsFieldExpressionWidget::currentFieldChanged()
 
 void QgsFieldExpressionWidget::updateLineEditStyle( const QString &expression )
 {
-  QPalette palette = mCombo->lineEdit()->palette();
+  QPalette palette( mComboPalette );
   if ( !isEnabled() )
   {
     palette.setColor( QPalette::Text, Qt::gray );
@@ -361,10 +363,6 @@ void QgsFieldExpressionWidget::updateLineEditStyle( const QString &expression )
     {
       palette.setColor( QPalette::Text, Qt::red );
     }
-    else
-    {
-      palette.setColor( QPalette::Text, Qt::black );
-    }
   }
   mCombo->lineEdit()->setPalette( palette );
 }

src/gui/qgsfieldexpressionwidget.h

@@ -249,6 +249,7 @@ class GUI_EXPORT QgsFieldExpressionWidget : public QWidget
     const QgsExpressionContextGenerator *mExpressionContextGenerator = nullptr;
     QString mBackupExpression;
     bool mAllowEvalErrors = false;
+    QPalette mComboPalette;
 
     friend class TestQgsFieldExpressionWidget;
 };