fix: add null checks to avoid NPE

Author: karenc-bq

wrapper/src/main/java/software/amazon/jdbc/plugin/readwritesplitting/ReadWriteSplittingPlugin.java

@@ -203,7 +203,7 @@ public <T, E extends Exception> T execute(
         if (this.writerConnection != null && !this.writerConnection.isClosed()) {
           this.writerConnection.clearWarnings();
         }
-        if (isConnectionUsable(this.readerConnection.get())) {
+        if (this.readerConnection != null && isConnectionUsable(this.readerConnection.get())) {
           this.readerConnection.get().clearWarnings();
         }
       } catch (final SQLException e) {
@@ -380,7 +380,7 @@ private void switchToWriterConnection(
       switchCurrentConnectionTo(this.writerConnection, writerHost);
     }
 
-    if (this.isReaderConnFromInternalPool) {
+    if (this.isReaderConnFromInternalPool && this.readerConnection != null) {
       this.closeConnectionIfIdle(this.readerConnection.get());
     }
 
@@ -412,13 +412,13 @@ private void switchToReaderConnection(final List<HostSpec> hosts)
       return;
     }
 
-    if (this.readerHostSpec != null && !hosts.contains(this.readerHostSpec)) {
+    if (this.readerConnection != null && this.readerHostSpec != null && !hosts.contains(this.readerHostSpec)) {
       // The old reader cannot be used anymore because it is no longer in the list of allowed hosts.
       closeConnectionIfIdle(this.readerConnection.get());
     }
 
     this.inReadWriteSplit = true;
-    if (!isConnectionUsable(this.readerConnection.get())) {
+    if (this.readerConnection == null || !isConnectionUsable(this.readerConnection.get())) {
       initializeReaderConnection(hosts);
     } else {
       try {
@@ -572,6 +572,6 @@ Connection getWriterConnection() {
   }
 
   Connection getReaderConnection() {
-    return this.readerConnection.get();
+    return this.readerConnection == null ? null : this.readerConnection.get();
   }
 }