check for None is essential commands

gprMax/input_cmds_singleuse.py

@@ -84,6 +84,7 @@ def process_singlecmds(singlecmds):
 
     # Spatial discretisation
     cmd = '#dx_dy_dz'
+    if singlecmds[cmd] is not None:
         tmp = [float(x) for x in singlecmds[cmd].split()]
         if len(tmp) != 3:
             raise CmdInputError(cmd + ' requires exactly three parameters')
@@ -94,6 +95,7 @@ def process_singlecmds(singlecmds):
 
     # Domain
     cmd = '#domain'
+    if singlecmds[cmd] is not None:
         tmp = [float(x) for x in singlecmds[cmd].split()]
         if len(tmp) != 3:
             raise CmdInputError(cmd + ' requires exactly three parameters')
@@ -111,6 +113,7 @@ def process_singlecmds(singlecmds):
 
     # Time window
     cmd = '#time_window'
+    if singlecmds[cmd] is not None:
         tmp = singlecmds[cmd].split()
         if len(tmp) != 1:
             raise CmdInputError(cmd + ' requires exactly one parameter to specify the time window. Either in seconds or number of iterations.')