Fixed NPE.

diff --git a/com.google.eclipse.protobuf.ui/src/com/google/eclipse/protobuf/ui/builder/protoc/ProtobufBuildParticipant.java b/com.google.eclipse.protobuf.ui/src/com/google/eclipse/protobuf/ui/builder/protoc/ProtobufBuildParticipant.java
index 312b6eb..d8e074f 100644
--- a/com.google.eclipse.protobuf.ui/src/com/google/eclipse/protobuf/ui/builder/protoc/ProtobufBuildParticipant.java
+++ b/com.google.eclipse.protobuf.ui/src/com/google/eclipse/protobuf/ui/builder/protoc/ProtobufBuildParticipant.java

@@ -75,7 +75,7 @@
       return null;
     }
     URI uri = resource.getURI();
-    if (!uri.isPlatformResource() && !uri.fileExtension().equals("proto")) {
+    if (!uri.isPlatformResource() && !"proto".equals(uri.fileExtension())) {
       return null;
     }
     return uri.toPlatformString(true);