Fix FinderInputFormatClass, allocate Map.

Author: jan-niestadt

engine/src/main/java/nl/inl/blacklab/index/FinderInputFormatClass.java

@@ -1,6 +1,7 @@
 package nl.inl.blacklab.index;
 
 import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
@@ -32,6 +33,7 @@ public InputFormat find(String formatIdentifier) {
      */
     private synchronized static Map<String, Class<? extends  DocIndexerLegacy>> getLegacyDocIndexers() {
         if (legacyDocIndexers == null) {
+            legacyDocIndexers = new HashMap<>();
             Reflections reflections = new Reflections("", new SubTypesScanner(false));
             for (Class<? extends  DocIndexerLegacy> cl: reflections.getSubTypesOf(DocIndexerLegacy.class)) {
                 String qualifiedName = cl.getName();