Fix PropertyExpression's expr annotation

src/main/java/ch/njol/skript/expressions/ExprTime.java

@@ -74,7 +74,7 @@ public Class<?>[] acceptChange(final ChangeMode mode) {
 
 	@Override
 	public void change(Event event, Object @Nullable [] delta, ChangeMode mode) {
-		if (getExpr() == null || delta == null)
+		if (delta == null)
 			return;
 
 		Object time = delta[0];

src/main/java/ch/njol/skript/expressions/base/PropertyExpression.java

@@ -1,15 +1,16 @@
 package ch.njol.skript.expressions.base;
 
-
 import ch.njol.skript.Skript;
 import ch.njol.skript.lang.Expression;
 import ch.njol.skript.lang.ExpressionType;
 import ch.njol.skript.lang.SkriptParser.ParseResult;
 import ch.njol.skript.lang.SyntaxElement;
 import ch.njol.skript.lang.util.SimpleExpression;
 import ch.njol.util.Kleenean;
+import com.google.common.base.Preconditions;
 import org.bukkit.event.Event;
-import org.jetbrains.annotations.Nullable;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.UnknownNullability;
 import org.skriptlang.skript.lang.converter.Converter;
 import org.skriptlang.skript.lang.converter.Converters;
 
@@ -95,15 +96,15 @@ public static <T> void registerDefault(Class<? extends Expression<T>> expression
 		Skript.registerExpression(expressionClass, type, ExpressionType.PROPERTY, getDefaultPatterns(property, fromType));
 	}
 
-	@Nullable
-	private Expression<? extends F> expr;
+	private @UnknownNullability Expression<? extends F> expr;
 
 	/**
 	 * Sets the expression this expression represents a property of. No reference to the expression should be kept.
 	 *
-	 * @param expr
+	 * @param expr The expression this expression represents a property of.
 	 */
-	protected final void setExpr(Expression<? extends F> expr) {
+	protected final void setExpr(@NotNull Expression<? extends F> expr) {
+		Preconditions.checkNotNull(expr, "The expr param cannot be null");
 		this.expr = expr;
 	}