Runtime error

After i finished inputting the tex, it noticed runtime error: tex is null.

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You have a problem.   If you post a screen capture of your blocks, perhaps someone can help you with the issue Steve.

If text is null, it probably means that you either did not type any text into a TextBox?  (it is not clear that is what you are trying to do) .  If you did that, you may not have pressed the Done key on the Android keyboard?  before you try to do whatever you try to do with the text in the TextBox.     

or is this something else?

How to capture an image of your Blocks screen

On Windows  Ctr + PrtScn  keyboard keys pressed at same time copies an image of the screen to the Windows Clipboard.  Open Paint and save the image by putting the contents of the clipboard into Paint. Save the image to your desktop or another folder as a png or jpg file.  On Win 7 and above you can use the Snipping tool (in Accessories) to capture an image of blocks directly from the screen.  Capture what you need and Save the file as to a folder on your PC.

On a Mac,  command-shift-3 (whole-screen) or command-shift-4 (drag a rectangle) to take a screenshot. The image file will appear on your desktop. 

To post an image to the forum:  Once you copy an image and named it something like capturedimage.png and you have put it in a folder on your PC;  find the blue Insert Image control in the forum text area (the icon is next to and left of the Link).  Press the icon.  A small screen opens..drag the capturedimage.png to the new screen; wait a few seconds while Google's forum loads the image to the text area.  Now the image is in the forum's text box area, ready to post.

Please save the images as a jpg or png file.   Other formats are possible but create problems for some forum members in reading your images.

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i am trying to create an app which has two sceen; when you press a botton ,then it will open another sceen and in this sceen you can input some message and then return the message to the first sceen.but when i finish inputting,it notice runtime error: tex is null.

here is the screenshot of my block.

    

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Global variables in App Inventor are only "global" for the single screen.  If you set a global variable on Screen 2, Screen1 won't see that.  

To pass things back and forth, you're best bet is to use a TinyDB, saving and restoring as necessary.

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