fix Django course creation

python/educational-core/src/com/jetbrains/edu/learning/stepic/EduStepicConnector.java

@@ -302,6 +302,9 @@ public class EduStepicConnector {
     final List<AnswerPlaceholder> placeholders = file.getAnswerPlaceholders();
     for (AnswerPlaceholder placeholder : placeholders) {
       final AnswerPlaceholderSubtaskInfo info = placeholder.getActiveSubtaskInfo();
+      if (info == null) {
+        continue;
+      }
       final int offset = placeholder.getOffset();
       final int length = placeholder.getLength();
       if (fileText.length() > offset + length) {