fix: cope with null arguments to custom exception constructors

src/main/java/edu/wisc/my/messages/exception/ExpiredMessageException.java

@@ -18,8 +18,11 @@ public class ExpiredMessageException
   private final LocalDateTime asOfWhen;
 
   public ExpiredMessageException(Message messageWithRequestedId, LocalDateTime asOfWhen) {
-    super("Message with id [" + messageWithRequestedId.getId() + "] is expired as of " + asOfWhen
-      .toString());
+    super("Message " +
+      ((messageWithRequestedId == null) ?
+        "" :
+        (" with id [" + messageWithRequestedId.getId() + "] "))
+      + "is expired as of " + asOfWhen);
     this.expiredMessage = messageWithRequestedId;
     this.asOfWhen = asOfWhen;
   }

src/main/java/edu/wisc/my/messages/exception/PrematureMessageException.java

@@ -16,10 +16,12 @@ public class PrematureMessageException
   private LocalDateTime asOfWhen;
 
   public PrematureMessageException(Message prematureMessage, LocalDateTime asOfWhen) {
-    super("Message with id " + prematureMessage.getId() + " is premature as of "
-      + asOfWhen.toString());
+    super("Message with id " +
+      ((null == prematureMessage) ?
+        "" :
+        prematureMessage.getId() + " ")
+      + "is premature as of " + asOfWhen);
     this.prematureMessage = prematureMessage;
     this.asOfWhen = asOfWhen;
   }
-
 }

src/test/java/edu/wisc/my/messages/exception/ExpiredMessageExceptionTest.java

@@ -0,0 +1,12 @@
+package edu.wisc.my.messages.exception;
+
+import org.junit.Test;
+
+public class ExpiredMessageExceptionTest {
+
+  @Test
+  public void copesWithNullConstructorArguments() {
+    new ExpiredMessageException(null, null);
+  }
+
+}

src/test/java/edu/wisc/my/messages/exception/PrematureMessageExceptionTest.java

@@ -0,0 +1,12 @@
+package edu.wisc.my.messages.exception;
+
+import org.junit.Test;
+
+public class PrematureMessageExceptionTest {
+
+  @Test
+  public void copesWithNullConstructorArguments() {
+    new PrematureMessageException(null, null);
+  }
+
+}