Fix null cause handling when closing protocol handler

Author: phaupt

mid-java-client-core/src/main/java/ch/swisscom/mid/client/impl/MIDClientImpl.java

@@ -93,15 +93,22 @@ public void close() {
         if (selectedProtocolHandler != null) {
             try {
                 selectedProtocolHandler.close();
-            } catch (Exception e) {
-                logClient.debug("ComProtocolHandler failed to close: {}: {}, cause: {}: {}",
-                                e.getClass().getSimpleName(),
-                                e.getLocalizedMessage(),
-                                e.getCause().getClass(),
-                                e.getCause().getLocalizedMessage());
-            }
-        }
-    }
+            } catch (Exception e) {
+                Throwable cause = e.getCause();
+                if (cause != null) {
+                    logClient.debug("ComProtocolHandler failed to close: {}: {}, cause: {}: {}",
+                                    e.getClass().getSimpleName(),
+                                    e.getLocalizedMessage(),
+                                    cause.getClass().getSimpleName(),
+                                    cause.getLocalizedMessage());
+                } else {
+                    logClient.debug("ComProtocolHandler failed to close: {}: {}, cause: <none>",
+                                    e.getClass().getSimpleName(),
+                                    e.getLocalizedMessage());
+                }
+            }
+        }
+    }
 
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