Fix a crash when asJson is called on an empty PolygonGeometry

src/core/geometry/qgscurvepolygon.cpp

@@ -402,22 +402,25 @@ QString QgsCurvePolygon::asJson( int precision ) const
   // GeoJSON does not support curves
   QString json = QStringLiteral( "{\"type\": \"Polygon\", \"coordinates\": [" );
 
-  std::unique_ptr< QgsLineString > exteriorLineString( exteriorRing()->curveToLine() );
-  QgsPointSequence exteriorPts;
-  exteriorLineString->points( exteriorPts );
-  json += QgsGeometryUtils::pointsToJSON( exteriorPts, precision ) + ", ";
-
-  std::unique_ptr< QgsLineString > interiorLineString;
-  for ( int i = 0, n = numInteriorRings(); i < n; ++i )
-  {
-    interiorLineString.reset( interiorRing( i )->curveToLine() );
-    QgsPointSequence interiorPts;
-    interiorLineString->points( interiorPts );
-    json += QgsGeometryUtils::pointsToJSON( interiorPts, precision ) + ", ";
-  }
-  if ( json.endsWith( QLatin1String( ", " ) ) )
+  if ( exteriorRing() )
   {
-    json.chop( 2 ); // Remove last ", "
+    std::unique_ptr< QgsLineString > exteriorLineString( exteriorRing()->curveToLine() );
+    QgsPointSequence exteriorPts;
+    exteriorLineString->points( exteriorPts );
+    json += QgsGeometryUtils::pointsToJSON( exteriorPts, precision ) + QLatin1String( ", " );
+
+    std::unique_ptr< QgsLineString > interiorLineString;
+    for ( int i = 0, n = numInteriorRings(); i < n; ++i )
+    {
+      interiorLineString.reset( interiorRing( i )->curveToLine() );
+      QgsPointSequence interiorPts;
+      interiorLineString->points( interiorPts );
+      json += QgsGeometryUtils::pointsToJSON( interiorPts, precision ) + QLatin1String( ", " );
+    }
+    if ( json.endsWith( QLatin1String( ", " ) ) )
+    {
+      json.chop( 2 ); // Remove last ", "
+    }
   }
   json += QLatin1String( "] }" );
   return json;