NullPointerException in Ed25519FieldElement.invert

[mckpm]

We occasionally see this crash in the field (haven't managed to reproduce):

#0. Crashed: main
       at net.i2p.crypto.eddsa.spec.EdDSANamedCurveTable.getByName(EdDSANamedCurveTable.java:67)
       at org.stellar.sdk.KeyPair.(KeyPair.java:29)
       at org.stellar.sdk.KeyPair.fromAccountId(KeyPair.java:104)
[...]

Fatal Exception: java.lang.ExceptionInInitializerError
       at net.i2p.crypto.eddsa.spec.EdDSANamedCurveTable.getByName(EdDSANamedCurveTable.java:67)
       at org.stellar.sdk.KeyPair.(KeyPair.java:29)
       at org.stellar.sdk.KeyPair.fromAccountId(KeyPair.java:104)
[...]

Caused by java.lang.NullPointerException: Attempt to invoke virtual method 'b.a.a.a.a.e b.a.a.a.a.e.g()' on a null object reference
       at net.i2p.crypto.eddsa.math.ed25519.Ed25519FieldElement.invert(Ed25519FieldElement.java:811)
       at net.i2p.crypto.eddsa.math.GroupElement.precompute(GroupElement.java:472)
       at net.i2p.crypto.eddsa.math.Curve.createPoint(Curve.java:78)
       at net.i2p.crypto.eddsa.spec.EdDSANamedCurveTable.(EdDSANamedCurveTable.java:43)
       at net.i2p.crypto.eddsa.spec.EdDSANamedCurveTable.getByName(EdDSANamedCurveTable.java:67)
       at org.stellar.sdk.KeyPair.(KeyPair.java:29)
       at org.stellar.sdk.KeyPair.fromAccountId(KeyPair.java:104)
[...]

[str4d]

invert() is being called here:

ed25519-java/src/net/i2p/crypto/eddsa/math/GroupElement.java

Lines 467 to 472 in 581c6a9

	// TODO-CR BR: check that this == base point when the method is called.
	Bi = this;
	for (int i = 0; i < 32; i++) {
	GroupElement Bij = Bi;
	for (int j = 0; j < 8; j++) {
	final FieldElement recip = Bij.Z.invert();

At the crash point, invert() is doing this:

ed25519-java/src/net/i2p/crypto/eddsa/math/ed25519/Ed25519FieldElement.java

Lines 806 to 812 in 581c6a9

	// 2^201 - 2^1
	t2 = t2.square();
	// 2^250 - 2^50
	for (int i = 1; i < 50; ++i) {
	t2 = t2.square();
	}

So my initial guess would be that t2.square() returns null at some point either before or during that loop, and the subsequent iteration then has t2 set to null, causing the NPE. However, Ed25519FieldElement.square() never returns null (it does deterministic maths and then calls a constructor, which can never return null but does potentially throw an IAE). So... I'm going to leave this open until someone else can reproduce this.