InputText not working
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* **What is the current behaviour?** The program crashes when I try to edit the `inputText`.

* **Please provide the steps to reproduce and if possible a minimal demo of the problem**:
```nim
import nimgl/imgui, nimgl/imgui/[impl_opengl, impl_glfw]
import nimgl/[opengl, glfw]

proc main() =
  doAssert glfwInit()

  glfwWindowHint(GLFWContextVersionMajor, 3)
  glfwWindowHint(GLFWContextVersionMinor, 3)
  glfwWindowHint(GLFWOpenglForwardCompat, GLFW_TRUE)
  glfwWindowHint(GLFWOpenglProfile, GLFW_OPENGL_CORE_PROFILE)
  glfwWindowHint(GLFWResizable, GLFW_FALSE)

  var w: GLFWWindow = glfwCreateWindow(1280, 720)
  if w == nil:
    quit(-1)

  w.makeContextCurrent()

  doAssert glInit()

  let context = igCreateContext()

  doAssert igGlfwInitForOpenGL(w, true)
  doAssert igOpenGL3Init()

  while not w.windowShouldClose:
    glfwPollEvents()

    igOpenGL3NewFrame()
    igGlfwNewFrame()
    igNewFrame()

    # Simple window
    igBegin("Hello, world!")
    igInputText("input", "text", 64u) # Error
    igEnd()
    # End simple window

    igRender()

    glClearColor(0.45f, 0.55f, 0.60f, 1.00f)
    glClear(GL_COLOR_BUFFER_BIT)

    igOpenGL3RenderDrawData(igGetDrawData())

    w.swapBuffers()

  igOpenGL3Shutdown()
  igGlfwShutdown()
  context.igDestroyContext()

  w.destroyWindow()
  glfwTerminate()

when isMainModule:
  main()
```

* **What is the expected behaviour?**: The `inputText` to be editable.

* **Please tell us about your environment**

  - Version: 1.3.2
  - OS: Linux amd64
  - Bindings: ImGui
  - Backend: CPP
  - Nim Version: 1.6.2
  - Compiler used: [GCC

* **Other information** (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, docs.gl, documentation etc)
```nim
Traceback (most recent call last)
/home/cristobal/.../trial.nim(56) trial
/home/cristobal/.../trial.nim(35) main
SIGSEGV: Illegal storage access. (Attempt to read from nil?)
Segmentation fault (core dumped)
Error: execution of an external program failed: '/home/cristobal/.../trial '
```