EditorContext::Begin: handle ImGuiEx::Canvas::Begin return at initial setup

if ImGuiEx::Canvas::Begin returns false (which can happen if the window is too small
and the canvas is Clipped), then the subsequent call to ImGuiEx::Canvas::End()
will trigger an IM_ASSERT

Author: pthom

imgui_node_editor.cpp

@@ -1123,9 +1123,9 @@ void ed::EditorContext::Begin(const char* id, const ImVec2& size)
 
     if (!m_IsInitialized)
     {
-        // Cycle canvas so it has a change to setup its size before settings are loaded
-        m_Canvas.Begin(id, canvasSize);
-        m_Canvas.End();
+        // Cycle canvas, so it has a chance to set up its size before settings are loaded
+        if (m_Canvas.Begin(id, canvasSize))
+            m_Canvas.End();
 
         LoadSettings();
         m_IsInitialized = true;