Add missing check for non-NULL frame_sp.

git-svn-id: https://llvm.org/svn/llvm-project/llvdb/trunk@137692 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/source/Target/Thread.cpp b/source/Target/Thread.cpp
index 11d28ac..5f51cd9 100644
--- a/source/Target/Thread.cpp
+++ b/source/Target/Thread.cpp
@@ -1116,10 +1116,13 @@
     if (GetProcess().GetTarget().GetDebugger().GetUseExternalEditor())
     {
         StackFrameSP frame_sp = GetStackFrameAtIndex(start_frame);
-        SymbolContext frame_sc(frame_sp->GetSymbolContext (eSymbolContextLineEntry));
-        if (frame_sc.line_entry.line != 0 && frame_sc.line_entry.file)
+        if (frame_sp)
         {
-            Host::OpenFileInExternalEditor (frame_sc.line_entry.file, frame_sc.line_entry.line);
+            SymbolContext frame_sc(frame_sp->GetSymbolContext (eSymbolContextLineEntry));
+            if (frame_sc.line_entry.line != 0 && frame_sc.line_entry.file)
+            {
+                Host::OpenFileInExternalEditor (frame_sc.line_entry.file, frame_sc.line_entry.line);
+            }
         }
     }