Fix #141339: Mantaflow: Clear Python errors when grids aren't available

Adding a Flow to a Mantaflow domain could sometimes cause a crash. This
was because the grids are allocated lazily, and as such, getting them
through `PyObject_GetAttrString` will fail when they are not yet
available. As the resulting Python errors were not cleared, Python could
be left in a bad state, leading to crashes. This is avoided by clearing
these errors before returning from `callPythonFunction` when such an
error is raised.

Ref !141364

intern/mantaflow/intern/MANTA_main.cpp

@@ -2029,6 +2029,7 @@ static PyObject *callPythonFunction(string varName, string functionName, bool is
 
   var = PyObject_GetAttrString(manta_main_module, varName.c_str());
   if (!var) {
+    PyErr_Clear();
     PyGILState_Release(gilstate);
     return nullptr;
   }
@@ -2037,12 +2038,17 @@ static PyObject *callPythonFunction(string varName, string functionName, bool is
 
   Py_DECREF(var);
   if (!func) {
+    PyErr_Clear();
     PyGILState_Release(gilstate);
     return nullptr;
   }
 
   if (!isAttribute) {
     returnedValue = PyObject_CallObject(func, nullptr);
+    if (returnedValue == nullptr) {
+      /* Print any unexpected errors, also clear them. */
+      PyErr_Print();
+    }
     Py_DECREF(func);
   }