fix NPE as reported in issues/1014

Signed-off-by: ceki <ceki@qos.ch>

Author: ceki

logback-core/src/main/java/ch/qos/logback/core/model/processor/DependencyDefinition.java

@@ -26,6 +26,10 @@
  */
 public class DependencyDefinition {
 
+    // depender depends on dependency (new terminology)
+    // dependency synonym dependee
+    // depender synonym dependent
+
     // OLD terminology: dependee (=dependency), dependent(=depender)
     //
     // NEW terminology: *dependent*: a component of type Model which depends on a *dependency*

logback-core/src/main/java/ch/qos/logback/core/util/VersionUtil.java

@@ -182,6 +182,11 @@ static public void checkForVersionEquality(Context context, String dependentVers
         // the dependent depends on the dependency
         addFoundVersionStatus(context, dependentName, dependentVersion);
 
+        if(dependencyVersion == null) {
+            String nullDependencyVersion = String.format("Failed to find version for %s.", dependencyName);
+            context.getStatusManager().add(new WarnStatus(nullDependencyVersion, context));
+            return;
+        }
         if (dependentVersion.equals(NA) || !dependentVersion.equals(dependencyVersion)) {
             addFoundVersionStatus(context, dependencyName, dependencyVersion);
             String discrepancyMsg = String.format("Versions of %s and %s are different!", dependencyName, dependentName);