Fix exception on first normalize

src/uk/co/majenko/audiobookrecorder/Sentence.java

@@ -2147,8 +2147,10 @@ public class Sentence extends BookTreeNode implements Cacheable {
     }
 
     public void clearPeakGainPoints() {
+		if (gainPoints != null) {
 	        gainPoints.clear();
 		}
+    }
 
     public void autoAddPeakGainPoints() {
         long ts = System.currentTimeMillis();