diff --git a/core/generator/flow_graph.py.mako b/core/generator/flow_graph.py.mako
index 64c1661..2daf678 100644
--- a/core/generator/flow_graph.py.mako
+++ b/core/generator/flow_graph.py.mako
@@ -161,6 +161,9 @@ gr.io_signature.makev(${len(io_sigs)}, ${len(io_sigs)}, [${', '.join(size_strs)}
 
         self._lock = threading.RLock()
 % endif
+% if not generate_options.startswith('hb'):
+        self.flowgraph_started = threading.Event()
+% endif
 ########################################################
 ##Create Parameters
 ##  Set the parameter to a property of self.
@@ -335,6 +338,7 @@ def main(top_block_cls=${class_name}, options=None):
     ${'snippets_main_after_init(tb)' if snippets['main_after_init'] else ''}
     % if flow_graph.get_option('run'):
     tb.start(${flow_graph.get_option('max_nouts') or ''})
+    tb.flowgraph_started.set()
     % endif
     ${'snippets_main_after_start(tb)' if snippets['main_after_start'] else ''}
     % if flow_graph.get_option('qt_qss_theme'):
@@ -367,6 +371,7 @@ def main(top_block_cls=${class_name}, options=None):
     ${'snippets_main_after_init(tb)' if snippets['main_after_init'] else ''}
     try:
         tb.start()
+        tb.flowgraph_started.set()
         ${'snippets_main_after_start(tb)' if snippets['main_after_start'] else ''}
         bokehgui.utils.run_server(tb, sizing_mode = "${flow_graph.get_option('sizing_mode')}",  widget_placement =  ${flow_graph.get_option('placement')}, window_size =  ${flow_graph.get_option('window_size')})
     finally:
@@ -393,6 +398,7 @@ def main(top_block_cls=${class_name}, options=None):
 
     % if flow_graph.get_option('run_options') == 'prompt':
     tb.start(${ flow_graph.get_option('max_nouts') or '' })
+    tb.flowgraph_started.set()
     ${'snippets_main_after_start(tb)' if snippets['main_after_start'] else ''}
     % for m in monitors:
     % if m.params['en'].get_value() == 'True':
@@ -407,6 +413,7 @@ def main(top_block_cls=${class_name}, options=None):
     ## ${'snippets_main_after_stop(tb)' if snippets['main_after_stop'] else ''}
     % elif flow_graph.get_option('run_options') == 'run':
     tb.start(${flow_graph.get_option('max_nouts') or ''})
+    tb.flowgraph_started.set()
     ${'snippets_main_after_start(tb)' if snippets['main_after_start'] else ''}
     % for m in monitors:
     % if m.params['en'].get_value() == 'True':
diff --git a/core/generator/top_block.py b/core/generator/top_block.py
index ff145c0..0322db5 100644
--- a/core/generator/top_block.py
+++ b/core/generator/top_block.py
@@ -163,8 +163,8 @@ class TopBlockGenerator(object):
             imports.append('import os')
             imports.append('import sys')
 
-        if fg.get_option('thread_safe_setters'):
-            imports.append('import threading')
+        # Used by thread_safe_setters and startup Event
+        imports.append('import threading')
 
         def is_duplicate(l):
             if (l.startswith('import') or l.startswith('from')) and l in seen: