How to fix Delphi error "Access viilation at adress 004190B8"?

Question

Delphi

Wrote a small test application in delphi using delphi 7. Never wrote in delphi before. The program solves the problem of synchronization of three processes. One generates random numbers and writes to the buffer, the second reads the numbers and calculates the square of each and writes them to another buffer, the third thread calculates the sum and outputs to Edit.
Window

When you click on the launch, the following message is displayed:

Already tried several methods from Google - it did not help. I'm running under Win 8.1

Answer 1

Victor Koltsov, of course, is right, but this is not the reason for this error. Judging by the "Write of address 0000000E" there is a record in the field of an object that does not exist, i.e. reference MyObject = nil. It is necessary to close the error window, then in the menu Search - Find Error... It is positioned on a line like MyObject.prop := value; or MyObject.SomeMethod(); carefully study the sequence of actions, understand why the assignment to MyObject := TMyObject.Create(); has not yet been completed by this point. (call constructor), well, fix it.
In addition, I will add something that is not obvious for a beginner: the third thread, which writes in Edit, is actually the first (or even zero, if counted from scratch), i.e. this is the main thread, the UI thread, because you need to work with VCL only from the main thread, but be careful not to hang the main UI thread when synchronizing its reading from the 2nd buffer with the thread that writes to this 2nd buffer.

Answer 2

It is necessary to synchronize the streams before reading / writing to the buffers, so that only 1 stream works with the buffer at a time.