>	ui_but_value_get(but=0x000001b9137e2e48) Line 2378
 	ui_but_is_pushed_ex(but=0x000001b9137e2e48, value=0x00000063c4bfea70) Line 2002
 	[Inline Frame] ui_but_update_select_flag() Line 2086
 	ui_but_update_ex(but=0x000001b9137e2e48, validate) Line 3506
 	[Inline Frame] ui_but_update() Line 3676
 	[Inline Frame] ui_but_update_and_icon_set() Line 4631
 	uiDefIconBut(block, type, retval, icon=546, x=0, y=0, width=20, height=20, poin=0x0000000000000068, min=1.00000000, max=0.000000000, a1=0.000000000, a2=0.000000000, tip=0x00007ff6cd3022b0) Line 5019
 	uiDefIconButBit(block=0x000001b9137b3a38, type, bit, retval, icon=546, x=0, y=0, width=20, height=20, poin=0x0000000000000068, min=1.00000000, max=0.000000000, a1=0.000000000, a2=0.000000000, tip=0x00007ff6cd3022b0) Line 5057
 	uiTemplateEditModeSelection(layout=0x000001b913436f68, C) Line 162
 	uiTemplateHeader3D_mode(layout=0x000001b913436f68, C=0x000001b904858c18) Line 238
 	RNA_function_call(C, reports, ptr, func, parms=0x00000063c4bfef88) Line 7494
 	pyrna_func_call(self, args, kw=0x0000000000000000) Line 6328
 	[External Code]	
 	bpy_class_call(C=0x000001b904858c18, ptr=0x00000063c4bff950, func=0x00007ff6ceeff300, parms=0x00000063c4bff968) Line 8513
 	header_draw(C=0x000001b904858c18, hdr=0x00000063c4bff9f0) Line 707
 	ED_region_header_layout(C=0x000001b904858c18, region=0x000001b9074367f8) Line 2872
 	ED_region_header(C=0x000001b904858c18, region=0x000001b9074367f8) Line 2935
 	ED_region_do_draw(C=0x000001b904858c18, region=0x000001b9074367f8) Line 543
 	wm_draw_window_offscreen(C=0x000001b904858c18, win=0x000001b906eaa2b8, stereo) Line 713
 	wm_draw_window(C=0x000001b904858c18, win=0x000001b906eaa2b8) Line 841
 	wm_draw_update(C=0x000001b904858c18) Line 1042
 	WM_main(C=0x000001b904858c18) Line 482
 	main(argc=1, UNUSED_argv_c=0x0000000000000000) Line 534
 	[External Code]

diff --git a/source/blender/editors/space_view3d/view3d_header.c b/source/blender/editors/space_view3d/view3d_header.c
index f2e42cd1725..3668c3060a2 100644
--- a/source/blender/editors/space_view3d/view3d_header.c
+++ b/source/blender/editors/space_view3d/view3d_header.c
@@ -136,11 +136,34 @@ void uiTemplateEditModeSelection(uiLayout *layout, struct bContext *C)

   UI_block_func_handle_set(block, do_view3d_header_buttons, NULL);

+  //////////////////////////////////////////////////////////////////////////////////////////
+  printf("Entered uiTemplateEditModeSelection in %s\n", CTX_data_mode_string(C));
+  //////////////////////////////////////////////////////////////////////////////////////////
+
   if (obedit && (obedit->type == OB_MESH)) {
     BMEditMesh *em = BKE_editmesh_from_object(obedit);
     uiLayout *row;
     uiBut *but;

+    //////////////////////////////////////////////////////////////////////////////////////////
+    if (BKE_editmesh_from_object(CTX_data_edit_object(C)) == NULL) {
+      printf("Context says we're in %s\n", CTX_data_mode_string(C));
+      printf("But there's no edit object editmesh\n");
+      printf("em=%x, &em-selectmode=%x\n", em, &em->selectmode);
+      printf("If we continue execution, we will crash in ui_but_value_get() Line 2378\n");
+      printf("Because it will try to access (short *)but->poin\n");
+      printf("which is BKE_editmesh_from_object(CTX_data_edit_object(C))->selectmode\n");
+      printf("and BKE_editmesh_from_object(CTX_data_edit_object(C)) is NULL\n");
+      printf("So I'm going to return now.\n");
+      printf("Notice how the viewport mode button is now broken.\n");
+      printf("Also all further redos will fail here.\n");
+      printf("An attempt to switch the mode (using tab or the button) may crash or recover\n");
+      printf("from this broken state.\n");
+      return;
+    }
+    //////////////////////////////////////////////////////////////////////////////////////////
+
     row = uiLayoutRow(layout, true);
     block = uiLayoutGetBlock(row);
     but = uiDefIconButBitS(

Entered uiTemplateEditModeSelection in objectmode
Entered uiTemplateEditModeSelection in mesh_edit
Entered uiTemplateEditModeSelection in objectmode
Entered uiTemplateEditModeSelection in mesh_edit
Entered uiTemplateEditModeSelection in objectmode
Entered uiTemplateEditModeSelection in mesh_edit
Entered uiTemplateEditModeSelection in mesh_edit
Context says we're in mesh_edit
But there's no edit object editmesh
em=0, &em-selectmode=68
If we continue execution, we will crash in ui_but_value_get() Line 2378
Because it will try to access (short *)but->poin
which is BKE_editmesh_from_object(CTX_data_edit_object(C))->selectmode
and BKE_editmesh_from_object(CTX_data_edit_object(C)) is NULL
So I'm going to return now.
Notice how the viewport mode button is now broken.
Also all further redos will fail here.
An attempt to switch the mode (using tab or the button) may crash or recover
from this broken state.