small changes on u_prev and u_prev_prev definition

ProcessLib/PhaseField/CreatePhaseFieldProcess.cpp

@@ -290,36 +290,49 @@ std::unique_ptr<Process> createPhaseFieldProcess(
 
     auto& properties = mesh.getProperties();
 
-    auto const u_prev_field_name =
+    auto u_prev_field_name =
         config.getConfigParameterOptional<std::string>("u_prev_field_name");
-
-    if (!properties.existsPropertyVector<double>(
-            u_prev_field_name, MeshLib::MeshItemType::Node, DisplacementDim))
+    if (u_prev_field_name)
     {
-        OGS_FATAL(
-            "No nodal double field with {} components and name {} "
-            "present "
-            "in "
-            "the mesh",
-            DisplacementDim, u_prev_field_name);
+        if (!properties.existsPropertyVector<double>(
+                u_prev_field_name, MeshLib::MeshItemType::Node,
+                DisplacementDim))
+        {
+            OGS_FATAL(
+                "No nodal double field with {} components and name {} "
+                "present "
+                "in "
+                "the mesh",
+                DisplacementDim, u_prev_field_name);
+        }
+        auto& u_prev = *properties.getPropertyVector<double>(
+            u_prev_field_name, MeshLib::MeshItemType::Node, DisplacementDim);
     }
-    auto& u_prev = *properties.getPropertyVector<double>(
-        u_prev_field_name, MeshLib::MeshItemType::Node, DisplacementDim);
-
-    auto const u_prev_prev_field_name =
+    else
+    {
+        // For u_prev, we can introduce a zeros vector as initial guess
+    };
+    auto u_prev_prev_field_name =
         config.getConfigParameterOptional<std::string>(
             "u_prev_prev_field_name");
-
-    auto& u_prev_prev =
-        properties.existsPropertyVector<double>(u_prev_prev_field_name,
-                                                MeshLib::MeshItemType::Node,
-                                                DisplacementDim)
-            ? *properties.getPropertyVector<double>(u_prev_prev_field_name,
+    if (u_prev_prev_field_name)
+    {
+        auto& u_prev_prev =
+            properties.existsPropertyVector<double>(u_prev_prev_field_name,
                                                     MeshLib::MeshItemType::Node,
                                                     DisplacementDim)
-            : *properties.createNewPropertyVector<double>(
-                  u_prev_prev_field_name, MeshLib::MeshItemType::Node,
-                  DisplacementDim);
+                ? *properties.getPropertyVector<double>(
+                      u_prev_prev_field_name,
+                      MeshLib::MeshItemType::Node,
+                      DisplacementDim)
+                : *properties.createNewPropertyVector<double>(
+                      u_prev_prev_field_name, MeshLib::MeshItemType::Node,
+                      DisplacementDim);
+    }
+    else
+    {
+        // For u_prev_prev, we can introduce a zeros vector as initial guess
+    };
 
     PhaseFieldProcessData<DisplacementDim> process_data{
         materialIDs(mesh),