Enable warnings about null references

Change-Id: Icff7dcb3ee72af0c87429510246747393ffd9527

.settings/org.eclipse.jdt.core.prefs

diff --git a/.settings/org.eclipse.jdt.core.prefs b/.settings/org.eclipse.jdt.core.prefs
index 2ece231..222bbe7 100644
--- a/.settings/org.eclipse.jdt.core.prefs
+++ b/.settings/org.eclipse.jdt.core.prefs
@@ -9,6 +9,8 @@
 org.eclipse.jdt.core.compiler.debug.sourceFile=generate
 org.eclipse.jdt.core.compiler.problem.assertIdentifier=error
 org.eclipse.jdt.core.compiler.problem.enumIdentifier=error
+org.eclipse.jdt.core.compiler.problem.nullReference=warning
+org.eclipse.jdt.core.compiler.problem.potentialNullReference=warning
 org.eclipse.jdt.core.compiler.problem.redundantSpecificationOfTypeArguments=warning
 org.eclipse.jdt.core.compiler.problem.unusedDeclaredThrownException=warning
 org.eclipse.jdt.core.compiler.problem.unusedDeclaredThrownExceptionExemptExceptionAndThrowable=enabled

src/main/java/com/google/gwtorm/jdbc/JdbcAccess.java

diff --git a/src/main/java/com/google/gwtorm/jdbc/JdbcAccess.java b/src/main/java/com/google/gwtorm/jdbc/JdbcAccess.java
index 553dc3e..ac4595a 100644
--- a/src/main/java/com/google/gwtorm/jdbc/JdbcAccess.java
+++ b/src/main/java/com/google/gwtorm/jdbc/JdbcAccess.java
@@ -14,6 +14,7 @@
 
 package com.google.gwtorm.jdbc;
 
+import com.google.common.base.Preconditions;
 import com.google.gwtorm.client.Key;
 import com.google.gwtorm.server.AbstractAccess;
 import com.google.gwtorm.server.Access;
@@ -351,6 +352,7 @@
         }
 
         if (0 < cnt) {
+          Preconditions.checkNotNull(ps);
           final int[] states = ps.executeBatch();
           if (states == null) {
             inserts = allInstances;