add better help for changing settings on the command-line

see #779

Engine/CLArgs.cpp

@@ -319,9 +319,11 @@ CLArgs::printUsage(const std::string& programName)
         "  --no-settings\n"
         "    When passed on the command-line, the %1 settings will not be restored\n"
         "    from the preferences file on disk so that %1 uses the default ones.\n"
-        "  --settings name=value\n"
+        "  --setting name=value\n"
         "    Sets the named %1 setting to the given value. This is done after loading\n"
         "    the settings and prior to executing Python commands or loading the project.\n"
+        "    For choice settings, the value may be an integer index for the option, or\n"
+        "    a quoted string, as in: --setting enableOpenGLRendering=\\\"enabled\\\"\n"
         "  -c [ --cmd ] \"PythonCommand\"\n"
         "    Execute custom Python code passed as a script prior to executing the Python\n"
         "    script or loading the project passed as parameter. This option may be used\n"

Engine/PyParameter.cpp

@@ -1665,7 +1665,8 @@ ChoiceParam::set(const QString& label)
         return;
     }
     std::string choiceID = label.toStdString();
-    if (knob->getHolder()->getApp()->isCreatingPythonGroup()) {
+    AppInstancePtr app = knob->getHolder()->getApp();
+    if (app && app->isCreatingPythonGroup()) {
         // Before Natron 2.2.3, all dynamic choice parameters for multiplane had a string parameter.
         // The string parameter had the same name as the choice parameter plus "Choice" appended.
         // If we found such a parameter, retrieve the string from it.