Fix crash when entering a digit on the SDL keyboard (#8388)

Fixes #8387.

frontend/device/sdl/device.lua

@@ -264,7 +264,7 @@ function Device:init()
                 self.window.left = ev.value.data1
                 self.window.top = ev.value.data2
             elseif ev.code == SDL_TEXTINPUT then
-                UIManager:sendEvent(Event:new("TextInput", ev.value))
+                UIManager:sendEvent(Event:new("TextInput", tostring(ev.value)))
             end
         end,
         hasClipboardText = function()

frontend/ui/widget/inputtext.lua

@@ -13,6 +13,7 @@ local TextBoxWidget = require("ui/widget/textboxwidget")
 local UIManager = require("ui/uimanager")
 local Utf8Proc = require("ffi/utf8proc")
 local VerticalGroup = require("ui/widget/verticalgroup")
+local dbg = require("dbg")
 local util = require("util")
 local _ = require("gettext")
 local Screen = Device.screen
@@ -728,6 +729,11 @@ function InputText:addChars(chars)
     self.charpos = self.charpos + #util.splitToChars(chars)
     self:initTextBox(table.concat(self.charlist), true)
 end
+dbg:guard(InputText, "addChars",
+    function(self, chars)
+        assert(type(chars) == "string",
+            "Wrong chars value type (expected string)!")
+    end)
 
 function InputText:delChar()
     if self.readonly or not self:isTextEditable(true) then