Bugfix in fields.py for GPU run without cupy (#4750)

* Bugfix in `fields.py` for GPU run without `cupy`

* apply suggestion from code review

Python/pywarpx/fields.py

@@ -47,6 +47,7 @@
     import cupy as cp
 except ImportError:
     cp = None
+from .LoadThirdParty import load_cupy
 
 try:
     from mpi4py import MPI as mpi
@@ -516,6 +517,12 @@ def __setitem__(self, index, value):
             if not isinstance(ic   , slice) or len(global_shape) < 4: global_shape[3:3] = [1]
             value3d.shape = global_shape
 
+            if libwarpx.libwarpx_so.Config.have_gpu:
+                # check if cupy is available for use
+                xp, cupy_status = load_cupy()
+                if cupy_status is not None:
+                    libwarpx.amr.Print(cupy_status)
+
         starts = [ixstart, iystart, izstart]
         stops = [ixstop, iystop, izstop]
         for mfi in self.mf:
@@ -526,7 +533,7 @@ def __setitem__(self, index, value):
                     slice_value = value3d[global_slices]
                     if libwarpx.libwarpx_so.Config.have_gpu:
                         # Copy data from host to device
-                        slice_value = cp.asarray(slice_value)
+                        slice_value = xp.asarray(slice_value)
                     mf_arr[block_slices] = slice_value
                 else:
                     mf_arr[block_slices] = value