Avoid crash with represent_value exp function

represent_value would crash when it was called via evaluate without any context specified.

src/core/expression/qgsexpressionfunction.cpp

@@ -3446,42 +3446,50 @@ static QVariant fcnRepresentValue( const QVariantList &values, const QgsExpressi
 {
   QVariant result;
   QString fieldName;
-  if ( !values.isEmpty() )
-  {
-    QgsExpressionNodeColumnRef *col = dynamic_cast<QgsExpressionNodeColumnRef *>( node->args()->at( 0 ) );
-    if ( col && ( values.size() == 1 || !values.at( 1 ).isValid() ) )
-      fieldName = col->name();
-    else if ( values.size() == 2 )
-      fieldName = QgsExpressionUtils::getStringValue( values.at( 1 ), parent );
-  }
-
-  QVariant value = values.at( 0 );
-
-  const QgsFields fields = context->fields();
-  int fieldIndex = fields.lookupField( fieldName );
 
-  if ( fieldIndex == -1 )
-  {
-    parent->setEvalErrorString( QCoreApplication::translate( "expression", "%1: Field not found %2" ).arg( QStringLiteral( "represent_value" ), fieldName ) );
-  }
-  else
+  if ( context )
   {
-    QgsVectorLayer *layer = QgsExpressionUtils::getVectorLayer( context->variable( "layer" ), parent );
-    const QgsEditorWidgetSetup setup = fields.at( fieldIndex ).editorWidgetSetup();
-    const QgsFieldFormatter *formatter = QgsApplication::fieldFormatterRegistry()->fieldFormatter( setup.type() );
+    if ( !values.isEmpty() )
+    {
+      QgsExpressionNodeColumnRef *col = dynamic_cast<QgsExpressionNodeColumnRef *>( node->args()->at( 0 ) );
+      if ( col && ( values.size() == 1 || !values.at( 1 ).isValid() ) )
+        fieldName = col->name();
+      else if ( values.size() == 2 )
+        fieldName = QgsExpressionUtils::getStringValue( values.at( 1 ), parent );
+    }
 
-    const QString cacheKey = QStringLiteral( "repvalfcn:%1:%2" ).arg( layer ? layer->id() : QStringLiteral( "[None]" ), fieldName );
+    QVariant value = values.at( 0 );
 
-    QVariant cache;
-    if ( !context->hasCachedValue( cacheKey ) )
+    const QgsFields fields = context->fields();
+    int fieldIndex = fields.lookupField( fieldName );
+
+    if ( fieldIndex == -1 )
     {
-      cache = formatter->createCache( layer, fieldIndex, setup.config() );
-      context->setCachedValue( cacheKey, cache );
+      parent->setEvalErrorString( QCoreApplication::translate( "expression", "%1: Field not found %2" ).arg( QStringLiteral( "represent_value" ), fieldName ) );
     }
     else
-      cache = context->cachedValue( cacheKey );
+    {
+      QgsVectorLayer *layer = QgsExpressionUtils::getVectorLayer( context->variable( "layer" ), parent );
+      const QgsEditorWidgetSetup setup = fields.at( fieldIndex ).editorWidgetSetup();
+      const QgsFieldFormatter *formatter = QgsApplication::fieldFormatterRegistry()->fieldFormatter( setup.type() );
+
+      const QString cacheKey = QStringLiteral( "repvalfcn:%1:%2" ).arg( layer ? layer->id() : QStringLiteral( "[None]" ), fieldName );
 
-    result = formatter->representValue( layer, fieldIndex, setup.config(), cache, value );
+      QVariant cache;
+      if ( !context->hasCachedValue( cacheKey ) )
+      {
+        cache = formatter->createCache( layer, fieldIndex, setup.config() );
+        context->setCachedValue( cacheKey, cache );
+      }
+      else
+        cache = context->cachedValue( cacheKey );
+
+      result = formatter->representValue( layer, fieldIndex, setup.config(), cache, value );
+    }
+  }
+  else
+  {
+    parent->setEvalErrorString( QCoreApplication::translate( "expression", "%1: function cannot be evaluated without a context." ).arg( QStringLiteral( "represent_value" ), fieldName ) );
   }
 
   return result;

tests/src/core/testqgsexpression.cpp

@@ -429,6 +429,15 @@ class TestQgsExpression: public QObject
       QCOMPARE( expression.evaluate( &context ).toString(), QStringLiteral( "one" ) );
 
       QgsExpression expression4( "represent_value('Class')" );
+      expression4.evaluate();
+      if ( expression4.hasParserError() )
+        qDebug() << expression4.parserErrorString();
+      QVERIFY( !expression4.hasParserError() );
+      if ( expression4.hasEvalError() )
+        qDebug() << expression4.evalErrorString();
+      QVERIFY( expression4.hasEvalError() );
+
+      expression4.prepare( &context );
       if ( expression4.hasParserError() )
         qDebug() << expression4.parserErrorString();
       QVERIFY( !expression4.hasParserError() );