Fix crash in ProfileInterrupt

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 2a9f10e56afb72662a0b29fbdee9367d42589502..a0226edef9fd262d2163b475a422ece793cc3567 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1015,10 +1015,14 @@ intptr_t Isolate::ProfileInterrupt() {
     // Profiler blocked for this isolate.
     return 0;
   }
-  if (message_handler()->paused_on_start() ||
-      message_handler()->paused_on_exit() ||
-      debugger()->IsPaused()) {
-    // Paused at start / exit / breakpoint. Don't tick.
+  if ((debugger() != NULL) && debugger()->IsPaused()) {
+    // Paused at breakpoint. Don't tick.
+    return 0;
+  }
+  if ((message_handler() != NULL) &&
+      (message_handler()->paused_on_start() ||
+       message_handler()->paused_on_exit())) {
+    // Paused at start / exit . Don't tick.
     return 0;
   }
   InterruptableThreadState* state = thread_state();