Resolved crash when mods make 2 techs require each other

tests/src/com/unciv/uniques/UniqueErrorTests.kt

@@ -0,0 +1,26 @@
+package com.unciv.uniques
+
+import com.unciv.models.ruleset.RulesetCache
+import com.unciv.testing.GdxTestRunner
+import org.junit.Test
+import org.junit.runner.RunWith
+
+
+@RunWith(GdxTestRunner::class)
+class UniqueErrorTests {
+    @Test
+    fun testCodependantTechs() {
+        RulesetCache.loadRulesets()
+        val ruleset = RulesetCache.getVanillaRuleset()
+
+        // Create a prerequisite loop
+        val techWithPrerequisites = ruleset.technologies.values.first { it.prerequisites.isNotEmpty() }
+        val prereq = ruleset.technologies[techWithPrerequisites.prerequisites.first()]!!
+        prereq.prerequisites.add(techWithPrerequisites.name)
+        ruleset.modOptions.isBaseRuleset = true
+
+        // Check mod links and ensure we don't get a crash, instead we get errors
+        val errors = ruleset.checkModLinks(false)
+        assert(errors.isNotOK())
+    }
+}