Why does Herbert say Pip should wait to tell Magwitch that he cannot accept his money?

From the text:

"See, then," said Herbert; "think of this! He comes here at the peril of his life, for the realization of his fixed idea. In the moment of realization, after all his toil and waiting, you cut the ground from under his feet, destroy his idea, and make his gains worthless to him. Do you see nothing that he might do, under the disappointment?"

"I have seen it, Herbert, and dreamed of it, ever since the fatal night of his arrival. Nothing has been in my thoughts so distinctly, as his putting himself in the way of being taken."

"Then you may rely upon it," said Herbert, "that there would be great danger of his doing it. That is his power over you as long as he remains in England, and that would be his reckless course if you forsook him."

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Herbert feels that if Pip tells Magwitch that he cannot accept his money he will take away Magwitch's dream. This would cause Magwitch to act rashly and put himself in danger of being caught by the authorities. Herbert's advice is for Pip to secret Magwitch out of England before telling Magwitch anything that would hurt him.