Gracefully handle that target temp may be null

Fixes eyal0#210

octoprint_PrintTimeGenius/__init__.py

@@ -183,7 +183,7 @@ def _allow_analysis(printer, settings):
   current_temp = all_temps[-1] # They are sorted so this is the most recent.
   elements_being_heated = 0
   for thermostat in current_temp.values():
-    if not isinstance(thermostat, collections.Mapping) or not 'actual' in thermostat or not 'target' in thermostat:
+    if not isinstance(thermostat, collections.Mapping) or not 'actual' in thermostat or not 'target' in thermostat or thermostat['target'] is None:
       continue
     if thermostat['target'] < 30:
       # This element is targeted for less than room temperature so ignore it.