Fix error drawing paint UI without a brush

Note that this removes image_paint.detect_data from
UnifiedPaintPanel.get_brush_mode, I can't see why it's needed,
it causes issues where the texture paint UI isn't used when it can be.

release/scripts/startup/bl_ui/properties_paint_common.py

@@ -29,8 +29,9 @@ class UnifiedPaintPanel:
     def get_brush_mode(context):
         """ Get the correct mode for this context. For any context where this returns None,
             no brush options should be displayed."""
+        mode = context.mode
 
-        if context.mode == 'PARTICLE':
+        if mode == 'PARTICLE':
             # Particle brush settings currently completely do their own thing.
             return None
 
@@ -54,14 +55,13 @@ class UnifiedPaintPanel:
                 if space_data.show_uvedit:
                     return 'UV_SCULPT'
                 return 'PAINT_2D'
-
-            if space_type in {'VIEW_3D', 'PROPERTIES'}:
-                if context.mode == 'PAINT_TEXTURE':
-                    if tool_settings.image_paint and tool_settings.image_paint.detect_data():
-                        return context.mode
+            elif space_type in {'VIEW_3D', 'PROPERTIES'}:
+                if mode == 'PAINT_TEXTURE':
+                    if tool_settings.image_paint:
+                        return mode
                     else:
                         return None
-                return context.mode
+                return mode
         return None
 
     @staticmethod
@@ -91,7 +91,7 @@ class UnifiedPaintPanel:
             return tool_settings.gpencil_paint
         elif mode in {'SCULPT_GPENCIL', 'WEIGHT_GPENCIL'}:
             return tool_settings.gpencil_sculpt
-        return False
+        return None
 
     @staticmethod
     def prop_unified(
@@ -160,6 +160,8 @@ class BrushSelectPanel(BrushPanel):
             row.column().template_ID(settings, "brush", new="brush.add")
         col = row.column()
         col.menu("VIEW3D_MT_brush_context_menu", icon='DOWNARROW_HLT', text="")
+
+        if brush is not None:
             col.prop(brush, "use_custom_icon", toggle=True, icon='FILE_IMAGE', text="")
 
             if brush.use_custom_icon:

release/scripts/startup/bl_ui/space_view3d_toolbar.py

@@ -400,6 +400,11 @@ class VIEW3D_PT_tools_brush_settings(Panel, View3DPaintBrushPanel):
     bl_context = ".paint_common"
     bl_label = "Brush Settings"
 
+    @classmethod
+    def poll(cls, context):
+        settings = cls.paint_settings(context)
+        return settings and settings.brush is not None
+
     def draw(self, context):
         layout = self.layout
 
@@ -491,8 +496,7 @@ class VIEW3D_PT_slots_projectpaint(View3DPanel, Panel):
     @classmethod
     def poll(cls, context):
         brush = context.tool_settings.image_paint.brush
-        ob = context.active_object
-        return (brush is not None and ob is not None)
+        return (brush is not None and context.active_object is not None)
 
     def draw(self, context):
         layout = self.layout
@@ -1373,6 +1377,7 @@ class VIEW3D_PT_tools_grease_pencil_brush_select(Panel, View3DPanel, GreasePenci
 
         if context.mode == 'PAINT_GPENCIL':
             brush = tool_settings.gpencil_paint.brush
+            if brush is not None:
                 gp_settings = brush.gpencil_settings
 
                 col.prop(brush, "use_custom_icon", toggle=True, icon='FILE_IMAGE', text="")