Throw IllegalStateException if the writer is not in a good state (#1097)

Author: sugmanue

core/src/main/java/software/amazon/smithy/java/core/serde/event/DefaultEventStreamWriter.java

@@ -102,12 +102,20 @@ public void setInitialEvent(IE initialEvent) {
 
     @Override
     public void addFrameProcessor(FrameProcessor<F> frameProcessor) {
+        if (this.encoderFactory == null) {
+            throw new IllegalStateException("encoderFactory is null. The encoder factory" +
+                    " has not been set or activate has been already called");
+        }
         this.encoderFactory =
                 encoderFactory.withFrameProcessor(Objects.requireNonNull(frameProcessor, "frameProcessor"));
     }
 
     @Override
     public void activate() {
+        if (this.encoderFactory == null) {
+            throw new IllegalStateException("encoderFactory is null. The encoder factory" +
+                    " has not been set or activate has been already called");
+        }
         setEventStreamEncoders();
         writeInitialEvent(initialEvent);
     }