I already read about this issue on stackoverflow but unfortunately haven't found the correct answer.
I use a ToggleButton and it shows me the following error in the rendering window:
The graphics preview in the layout editor may not be accurate: 
- Different corner sizes are not supported in Path.addRoundRect. (Ignore for this session) 
- Path.isConvex is not supported. (Ignore for this session)
The terminal shows the following error message:
E/dalvikvm: Could not find class 'android.widget.ThemedSpinnerAdapter', referenced from method android.support.v7.widget.AppCompatSpinner$DropDownAdapter.<init>
When I set the api level in the rendering window to 19 instead of 23 the "Rendering problem" window won't appear but the error still occurs in the app when running on a device with Android 4.4.2.
This is my simple ToggleButton:
<ToggleButton
                android:id="@+id/editToggleKunde6"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:textOn="@string/kunde_bearbeiten_txt1"
                android:textOff="@string/kunde_bearbeiten_txt2"
                android:checked="true"
                />
Thanks for any idea!