Fix for HER-1962 NPE from missing sheet

* SheetOverlaysManager.java
    getOverlayMap(String) - return null if sheet missing instead of triggering npe
* KeyedProperties.java
    get(String) - check for null return value from getOverlayMap() and log warning

commons/src/main/java/org/archive/spring/KeyedProperties.java

@@ -24,6 +24,7 @@ import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.logging.Logger;
 
 /**
  * Map for storing overridable properties. 
@@ -36,6 +37,9 @@ import java.util.concurrent.ConcurrentHashMap;
  */
 public class KeyedProperties extends ConcurrentHashMap<String,Object> {
     private static final long serialVersionUID = 3403222335436162778L;
+    
+    private static final Logger logger = Logger.getLogger(KeyedProperties.class.getName());
+
     /** the alternate global property-paths leading to this map 
      * TODO: consider if deterministic ordered list is important */
     HashSet<String> externalPaths = new HashSet<String>(); 
@@ -61,11 +65,15 @@ public class KeyedProperties extends ConcurrentHashMap<String,Object> {
             for(int j = ocontext.getOverlayNames().size()-1; j>=0; j--) {
                 String name = ocontext.getOverlayNames().get(j);
                 Map<String,Object> m = ocontext.getOverlayMap(name);
-                for(String ok : getOverrideKeys(key)) {
-                    Object val = m.get(ok);
-                    if(val!=null) {
-                        return val;
+                if (m != null) { 
+                    for(String ok : getOverrideKeys(key)) {
+                        Object val = m.get(ok);
+                        if(val!=null) {
+                            return val;
+                        }
                     }
+                } else {
+                    logger.warning("sheet '" + name + "' should apply but there is no such sheet!");
                 }
             }
         }

engine/src/main/java/org/archive/crawler/spring/SheetOverlaysManager.java

@@ -169,7 +169,12 @@ BeanFactoryAware, OverlayMapsSource, ApplicationListener {
      * @see org.archive.spring.OverlayMapsSource#getOverlayMap(java.lang.String)
      */
     public Map<String, Object> getOverlayMap(String name) {
-        return sheetsByName.get(name).getMap();
+        Sheet sheet = sheetsByName.get(name);
+        if (sheet != null) {
+            return sheet.getMap();
+        } else {
+            return null;
+        }
     }
 
     /**