Sketcher: Add check for validity of geometry when trimming

Possibly solves #19425.

src/Mod/Sketcher/App/SketchObject.cpp

@@ -3324,6 +3324,11 @@ int SketchObject::trim(int GeoId, const Base::Vector3d& point)
     // FIXME: we should be able to transfer these to new curves smoothly
     // auto geo = getGeometry(GeoId);
     const auto* geoAsCurve = getGeometry<Part::GeomCurve>(GeoId);
+
+    if (geoAsCurve == nullptr) {
+        return -1;
+    }
+
     bool isOriginalCurveConstruction = GeometryFacade::getConstruction(geoAsCurve);
 
     //******************* Step A => Detection of intersection - Common to all Geometries