Fix a crash in TimeUnitConversionChecker

Motivated by

```
thread.join((timeUnit != null ? timeUnit : TimeUnit.MILLISECONDS).toMillis(time));
```

https://github.com/apache/httpcomponents-client/blame/160bac0cbb2b388018c4e794c120ba719ddd2887/httpclient5/src/main/java/org/apache/hc/client5/http/impl/IdleConnectionEvictor.java#L98

PiperOrigin-RevId: 567037317

core/src/main/java/com/google/errorprone/bugpatterns/time/TimeUnitConversionChecker.java

@@ -32,6 +32,7 @@
 import com.sun.source.tree.ExpressionTree;
 import com.sun.source.tree.MethodInvocationTree;
 import com.sun.source.tree.Tree;
+import com.sun.tools.javac.code.Symbol;
 import java.util.concurrent.TimeUnit;
 
 /** Check for problematic or suspicious TimeUnit conversion calls. */
@@ -73,7 +74,11 @@ public Description matchMethodInvocation(MethodInvocationTree tree, VisitorState
     // TimeUnit SECONDS = TimeUnit.MINUTES;
     // long about2500 = SECONDS.toSeconds(42);
     // but... I think that's bad enough to ignore here :)
-    String timeUnitName = ASTHelpers.getSymbol(receiverOfConversion).getSimpleName().toString();
+    Symbol receiverOfConversionSymbol = ASTHelpers.getSymbol(receiverOfConversion);
+    if (receiverOfConversionSymbol == null) {
+      return Description.NO_MATCH;
+    }
+    String timeUnitName = receiverOfConversionSymbol.getSimpleName().toString();
     Optional<TimeUnit> receiver = Enums.getIfPresent(TimeUnit.class, timeUnitName);
     if (!receiver.isPresent()) {
       return Description.NO_MATCH;

core/src/test/java/com/google/errorprone/bugpatterns/time/TimeUnitConversionCheckerTest.java

@@ -168,4 +168,18 @@ public void largeUnitToSmallUnit_succeeds() {
             "}")
         .doTest();
   }
+
+  @Test
+  public void receiverExpression() {
+    helper
+        .addSourceLines(
+            "TestClass.java",
+            "import java.util.concurrent.TimeUnit;",
+            "public class TestClass {",
+            "  long f(TimeUnit timeUnit, long time) {",
+            "    return (timeUnit != null ? timeUnit : TimeUnit.MILLISECONDS).toMillis(time);",
+            "  }",
+            "}")
+        .doTest();
+  }
 }